Make DetectCollisions read-only (separate detection from modification)

## Overview
Phase 2: Separate collision detection from state modification to enable safe, composable collision resolution.

## Current Problem
DetectCollisions (collision.go:38-111) modifies database state during detection:
- Line 83-86: Deletes issues when content matches but ID differs
- This violates separation of concerns
- Causes race conditions when processing multiple issues
- Makes contentToDBIssue map stale after first deletion
- Partial failures leave DB in inconsistent state

## Solution
Make DetectCollisions purely read-only. Move all modifications to a separate ApplyCollisionResolution function.

## Implementation Tasks

### 1. Add RenameDetail to CollisionResult
File: internal/storage/sqlite/collision.go
```go
type CollisionResult struct {
    ExactMatches []string
    Collisions   []*CollisionDetail
    NewIssues    []string
    Renames      []*RenameDetail  // NEW
}

type RenameDetail struct {
    OldID string        // ID in database
    NewID string        // ID in incoming
    Issue *types.Issue  // The issue with new ID
}
```

### 2. Remove deletion from DetectCollisions
Replace lines 83-86:
```go
// OLD (DELETE THIS):
if err := s.DeleteIssue(ctx, dbMatch.ID); err != nil {
    return nil, fmt.Errorf("failed to delete renamed issue...")
}

// NEW (ADD THIS):
result.Renames = append(result.Renames, &RenameDetail{
    OldID: dbMatch.ID,
    NewID: incoming.ID,
    Issue: incoming,
})
continue  // Don't mark as NewIssue yet
```

### 3. Create ApplyCollisionResolution function
New function to apply all modifications atomically:
```go
func ApplyCollisionResolution(ctx context.Context, s *SQLiteStorage,
    result *CollisionResult, mapping map[string]string) error {

    // Phase 1: Handle renames (delete old IDs)
    for _, rename := range result.Renames {
        if err := s.DeleteIssue(ctx, rename.OldID); err != nil {
            return fmt.Errorf("failed to delete renamed issue %s: %w",
                rename.OldID, err)
        }
    }

    // Phase 2: Create new IDs (from mapping)
    // Phase 3: Update references
    return nil
}
```

### 4. Update callers to use two-phase approach
File: internal/importer/importer.go (handleCollisions)
```go
// Phase 1: Detect (read-only)
collisionResult, err := sqlite.DetectCollisions(ctx, sqliteStore, issues)

// Phase 2: Resolve (compute mapping)
mapping, err := sqlite.ResolveNWayCollisions(ctx, sqliteStore, collisionResult)

// Phase 3: Apply (modify DB)
err = sqlite.ApplyCollisionResolution(ctx, sqliteStore, collisionResult, mapping)
```

### 5. Update tests
- Verify DetectCollisions doesn't modify DB
- Test ApplyCollisionResolution separately
- Add test for rename detection without modification

## Acceptance Criteria
- DetectCollisions performs zero writes to database
- DetectCollisions returns RenameDetail entries for content matches
- ApplyCollisionResolution handles all modifications
- All existing tests still pass
- New test verifies read-only detection
- contentToDBIssue map stays consistent throughout detection

## Files to Modify
- internal/storage/sqlite/collision.go (DetectCollisions, new function)
- internal/importer/importer.go (handleCollisions caller)
- internal/storage/sqlite/collision_test.go (add tests)

## Testing
- Unit test: DetectCollisions with content match doesn't delete DB issue
- Unit test: RenameDetail correctly populated
- Unit test: ApplyCollisionResolution applies renames
- Integration test: Full flow still works end-to-end

## Risk Mitigation
This is a significant refactor of core collision logic. Recommend:
1. Add comprehensive tests before modifying
2. Use feature flag to enable/disable new behavior
3. Test thoroughly with TestTwoCloneCollision first


Implement global N-way collision resolution algorithm

## Overview
Phase 3: Replace pairwise collision resolution with global N-way resolution that produces deterministic results regardless of sync order.

## Current Problem
ScoreCollisions (collision.go:228) compares issues pairwise:
```go
collision.RemapIncoming = existingHash < incomingHash
```

This works for 2-way but fails for 3+ way because:
- Each clone makes local decisions without global context
- No guarantee intermediate states are consistent
- Remapping decisions depend on sync order
- Can't detect transitive remap chains (test-1 → test-2 → test-3)

## Solution
Implement global resolution that:
1. Collects ALL versions of same logical issue
2. Sorts by content hash (deterministic)
3. Assigns sequential IDs based on sorted order
4. All clones converge to same assignments

## Implementation Tasks

### 1. Create ResolveNWayCollisions function
File: internal/storage/sqlite/collision.go

Replace ScoreCollisions with:
```go
// ResolveNWayCollisions handles N-way collisions deterministically.
// Groups all versions with same base ID, sorts by content hash,
// assigns sequential IDs. Returns mapping of old ID → new ID.
func ResolveNWayCollisions(ctx context.Context, s *SQLiteStorage,
    collisions []*CollisionDetail, incoming []*types.Issue) (map[string]string, error) {

    if len(collisions) == 0 {
        return make(map[string]string), nil
    }

    // Group by base ID pattern (e.g., test-1, test-2 → base "test-1")
    groups := groupCollisionsByBaseID(collisions)

    idMapping := make(map[string]string)

    for baseID, versions := range groups {
        // 1. Collect all unique versions by content hash
        uniqueVersions := deduplicateVersionsByContentHash(versions)

        // 2. Sort by content hash (deterministic!)
        sort.Slice(uniqueVersions, func(i, j int) bool {
            return uniqueVersions[i].ContentHash < uniqueVersions[j].ContentHash
        })

        // 3. Assign sequential IDs based on sorted order
        prefix := extractPrefix(baseID)
        baseNum := extractNumber(baseID)

        for i, version := range uniqueVersions {
            targetID := fmt.Sprintf("%s-%d", prefix, baseNum+i)

            // Map this version to its deterministic ID
            if version.ID != targetID {
                idMapping[version.ID] = targetID
            }
        }
    }

    return idMapping, nil
}
```

### 2. Implement helper functions

```go
// groupCollisionsByBaseID groups collisions by their logical base ID
func groupCollisionsByBaseID(collisions []*CollisionDetail) map[string][]*types.Issue {
    groups := make(map[string][]*types.Issue)
    for _, c := range collisions {
        baseID := c.ID  // All share same ID (that's why they collide)
        groups[baseID] = append(groups[baseID], c.ExistingIssue, c.IncomingIssue)
    }
    return groups
}

// deduplicateVersionsByContentHash keeps one issue per unique content hash
func deduplicateVersionsByContentHash(issues []*types.Issue) []*types.Issue {
    seen := make(map[string]*types.Issue)
    for _, issue := range issues {
        if _, found := seen[issue.ContentHash]; !found {
            seen[issue.ContentHash] = issue
        }
    }
    result := make([]*types.Issue, 0, len(seen))
    for _, issue := range seen {
        result = append(result, issue)
    }
    return result
}
```

### 3. Update handleCollisions in importer
File: internal/importer/importer.go

Replace ScoreCollisions call with:
```go
// OLD:
if err := sqlite.ScoreCollisions(ctx, sqliteStore, collisionResult.Collisions, allExistingIssues); err != nil {
    return nil, fmt.Errorf("failed to score collisions: %w", err)
}

// NEW:
idMapping, err := sqlite.ResolveNWayCollisions(ctx, sqliteStore,
    collisionResult.Collisions, issues)
if err != nil {
    return nil, fmt.Errorf("failed to resolve collisions: %w", err)
}
```

### 4. Update RemapCollisions
RemapCollisions currently uses collision.RemapIncoming field. Update to use idMapping directly:
- Remove RemapIncoming logic
- Use idMapping to determine what to remap
- Simplify to just apply the computed mapping

### 5. Add comprehensive tests

Test cases:
1. 3-way collision with different content → 3 sequential IDs
2. 3-way collision with 2 identical content → 2 IDs (dedupe works)
3. Sync order independence (A→B→C vs C→A→B produce same result)
4. Content hash ordering is respected
5. Works with 5+ clones

## Acceptance Criteria
- ResolveNWayCollisions implemented and replaces ScoreCollisions
- Groups all versions of same ID together
- Deduplicates by content hash
- Sorts by content hash deterministically
- Assigns sequential IDs starting from base ID
- Returns complete mapping (old ID → new ID)
- All clones converge to same ID assignments
- Works for arbitrary N-way collisions
- TestThreeCloneCollision passes (or gets much closer)

## Files to Modify
- internal/storage/sqlite/collision.go (new function, helpers)
- internal/importer/importer.go (call new function)
- internal/storage/sqlite/collision_test.go (comprehensive tests)

## Testing Strategy

### Unit Tests
- groupCollisionsByBaseID correctly groups
- deduplicateVersionsByContentHash removes duplicates
- Sorting by hash is stable and deterministic
- Sequential ID assignment is correct

### Integration Tests
- 3-way collision resolves to 3 issues
- Sync order doesn't affect final IDs
- Content hash ordering determines winner

### Property Tests
- For any N clones with same content, all converge to same IDs
- Idempotent: running resolution twice produces same result

## Dependencies
- Requires bd-95 (ContentHash field) to be completed first
- Requires bd-96 (read-only detection) for clean integration

## Notes
This is the core algorithm that enables convergence. The key insight:
**Sort by content hash globally, not pairwise comparison.**


Implement content-first idempotent import

## Overview
Phase 4: Refactor import to be content-first and idempotent, ensuring importing same JSONL multiple times always converges correctly.

## Current Problem
Current import is ID-first:
1. Look up by ID
2. If exists, update
3. If not exists, create

This causes issues when:
- Same content arrives with different IDs (renames not detected)
- Multiple rounds of import needed for convergence
- Import order affects final state

## Solution
Make import content-first and idempotent:
1. Hash all incoming and existing issues
2. Match by content hash first (detect renames)
3. Handle ID conflicts second (using global resolution)
4. Ensure importing same data multiple times = no-op

## Implementation Tasks

### 1. Refactor ImportIssues to be content-first
File: internal/importer/importer.go

```go
func ImportIssues(ctx context.Context, dbPath string, store storage.Storage,
    issues []*types.Issue, opts Options) (*Result, error) {

    result := &Result{...}

    sqliteStore, needCloseStore, err := getOrCreateStore(ctx, dbPath, store)
    if err != nil {
        return nil, err
    }
    if needCloseStore {
        defer func() { _ = sqliteStore.Close() }()
    }

    // Phase 1: Compute content hashes for all incoming issues
    for _, issue := range issues {
        issue.ContentHash = issue.ComputeContentHash()
    }

    // Phase 2: Build content hash maps
    incomingByHash := buildHashMap(issues)
    dbIssues, _ := sqliteStore.SearchIssues(ctx, "", types.IssueFilter{})
    dbByHash := buildHashMap(dbIssues)
    dbByID := buildIDMap(dbIssues)

    // Phase 3: Content-first matching
    var newIssues []*types.Issue
    var idConflicts []*CollisionDetail

    for hash, incoming := range incomingByHash {
        if existing, found := dbByHash[hash]; found {
            // Same content exists
            if existing.ID == incoming.ID {
                // Exact match - idempotent case
                result.Unchanged++
            } else {
                // Same content, different ID - rename detected
                // Delete old ID, keep new ID (incoming is canonical)
                if err := handleRename(ctx, sqliteStore, existing, incoming); err != nil {
                    return nil, err
                }
                result.Updated++
            }
        } else {
            // New content - check for ID collision
            if existingWithID, found := dbByID[incoming.ID]; found {
                // ID exists but different content - collision
                idConflicts = append(idConflicts, &CollisionDetail{
                    ID:            incoming.ID,
                    IncomingIssue: incoming,
                    ExistingIssue: existingWithID,
                })
            } else {
                // Truly new issue
                newIssues = append(newIssues, incoming)
            }
        }
    }

    // Phase 4: Resolve ID conflicts using global algorithm
    if len(idConflicts) > 0 {
        if !opts.ResolveCollisions {
            return nil, fmt.Errorf("collision detected")
        }

        idMapping, err := sqlite.ResolveNWayCollisions(ctx, sqliteStore,
            idConflicts, issues)
        if err != nil {
            return nil, err
        }

        if err := applyIDMapping(ctx, sqliteStore, idMapping); err != nil {
            return nil, err
        }

        result.IDMapping = idMapping
        result.Collisions = len(idConflicts)
    }

    // Phase 5: Create new issues
    if len(newIssues) > 0 {
        if err := sqliteStore.CreateIssues(ctx, newIssues, "import"); err != nil {
            return nil, err
        }
        result.Created = len(newIssues)
    }

    // Phase 6: Import dependencies, labels, comments (existing logic)
    // ...

    return result, nil
}
```

### 2. Implement helper functions

```go
// buildHashMap creates a map of content hash → issue
func buildHashMap(issues []*types.Issue) map[string]*types.Issue {
    result := make(map[string]*types.Issue)
    for _, issue := range issues {
        result[issue.ContentHash] = issue
    }
    return result
}

// buildIDMap creates a map of ID → issue
func buildIDMap(issues []*types.Issue) map[string]*types.Issue {
    result := make(map[string]*types.Issue)
    for _, issue := range issues {
        result[issue.ID] = issue
    }
    return result
}

// handleRename handles content match with different IDs
func handleRename(ctx context.Context, s *SQLiteStorage,
    existing *types.Issue, incoming *types.Issue) error {

    // Delete old ID
    if err := s.DeleteIssue(ctx, existing.ID); err != nil {
        return fmt.Errorf("failed to delete old ID %s: %w", existing.ID, err)
    }

    // Create with new ID
    if err := s.CreateIssue(ctx, incoming, "import-rename"); err != nil {
        return fmt.Errorf("failed to create renamed issue %s: %w",
            incoming.ID, err)
    }

    // Update references from old ID to new ID
    idMapping := map[string]string{existing.ID: incoming.ID}
    return updateReferences(ctx, s, idMapping)
}
```

### 3. Add idempotency tests

Test cases:
1. Import same JSONL twice → second import reports all Unchanged
2. Import, modify DB, import again → reports Updated
3. Import with rename, import again → idempotent
4. Import with collision resolution, import again → idempotent

### 4. Update handleCollisions to use new flow
Current handleCollisions in importer.go needs to be updated to:
- Use content-first matching
- Call new ResolveNWayCollisions
- Apply results using ApplyCollisionResolution

## Acceptance Criteria
- Import matches by content hash before checking IDs
- Importing same JSONL multiple times is idempotent (reports Unchanged)
- Rename detection works (same content, different ID)
- ID conflicts resolved using global algorithm
- Result.Unchanged correctly tracks idempotent imports
- TestThreeCloneCollision passes
- All existing import tests still pass

## Testing Strategy

### Unit Tests
- buildHashMap correctly indexes by content hash
- buildIDMap correctly indexes by ID
- handleRename deletes old, creates new, updates references

### Integration Tests
- Import same data twice → idempotent
- Import renamed issue → handled correctly
- Import with collision → resolved globally
- Final pull after 3-way collision → all clones converge

### Property Tests
- Idempotency: Import(x); Import(x) ≡ Import(x)
- Commutativity: Import(a); Import(b) ≡ Import(b); Import(a) (for non-colliding issues)
- Convergence: After N rounds of sync, all clones identical

## Files to Modify
- internal/importer/importer.go (major refactor of ImportIssues)
- internal/importer/importer_test.go (new tests)
- cmd/bd/import_bug_test.go (update for new behavior)

## Dependencies
- Requires bd-95 (ContentHash field)
- Requires bd-96 (read-only detection)
- Requires bd-97 (global resolution)

## Risk Mitigation
Major refactor of import logic. Recommend:
1. Comprehensive tests before modifying
2. Feature flag to enable/disable
3. Keep old import code path for rollback
4. Test with all existing import tests
5. Manual testing with real repositories

## Success Metrics
After this phase:
- TestThreeCloneCollision should PASS
- All clones converge after final pull
- Import is demonstrably idempotent
- No data loss in N-way scenarios


Add TestNWayCollision for 5+ clones

## Overview
Add comprehensive tests for N-way (5+) collision resolution to verify the solution scales beyond 3 clones.

## Purpose
While TestThreeCloneCollision validates the basic N-way case, we need to verify:
1. Solution scales to arbitrary N
2. Performance is acceptable with more clones
3. Convergence time is bounded
4. No edge cases in larger collision groups

## Implementation Tasks

### 1. Create TestFiveCloneCollision
File: beads_twoclone_test.go (or new beads_nway_test.go)

```go
func TestFiveCloneCollision(t *testing.T) {
    // Test with 5 clones creating same ID with different content
    // Verify all 5 clones converge after sync rounds

    t.Run("SequentialSync", func(t *testing.T) {
        testNCloneCollision(t, 5, "A", "B", "C", "D", "E")
    })

    t.Run("ReverseSync", func(t *testing.T) {
        testNCloneCollision(t, 5, "E", "D", "C", "B", "A")
    })

    t.Run("RandomSync", func(t *testing.T) {
        testNCloneCollision(t, 5, "C", "A", "E", "B", "D")
    })
}
```

### 2. Implement generalized testNCloneCollision
Generalize the 3-clone test to handle arbitrary N:

```go
func testNCloneCollision(t *testing.T, numClones int, syncOrder ...string) {
    t.Helper()

    if len(syncOrder) != numClones {
        t.Fatalf("syncOrder length (%d) must match numClones (%d)",
            len(syncOrder), numClones)
    }

    tmpDir := t.TempDir()

    // Setup remote and N clones
    remoteDir := setupBareRepo(t, tmpDir)
    cloneDirs := make(map[string]string)

    for i := 0; i < numClones; i++ {
        name := string(rune('A' + i))
        cloneDirs[name] = setupClone(t, tmpDir, remoteDir, name)
    }

    // Each clone creates issue with same ID but different content
    for name, dir := range cloneDirs {
        createIssue(t, dir, fmt.Sprintf("Issue from clone %s", name))
    }

    // Sync in specified order
    for _, name := range syncOrder {
        syncClone(t, cloneDirs[name], name)
    }

    // Final pull for convergence
    for name, dir := range cloneDirs {
        finalPull(t, dir, name)
    }

    // Verify all clones have all N issues
    expectedTitles := make(map[string]bool)
    for i := 0; i < numClones; i++ {
        name := string(rune('A' + i))
        expectedTitles[fmt.Sprintf("Issue from clone %s", name)] = true
    }

    for name, dir := range cloneDirs {
        titles := getTitles(t, dir)
        if !compareTitleSets(titles, expectedTitles) {
            t.Errorf("Clone %s missing issues: expected %v, got %v",
                name, expectedTitles, titles)
        }
    }

    t.Log("✓ All", numClones, "clones converged successfully")
}
```

### 3. Add performance benchmarks
Test convergence time and memory usage:

```go
func BenchmarkNWayCollision(b *testing.B) {
    for _, n := range []int{3, 5, 10, 20} {
        b.Run(fmt.Sprintf("N=%d", n), func(b *testing.B) {
            for i := 0; i < b.N; i++ {
                // Run N-way collision and measure time
                testNCloneCollisionBench(b, n)
            }
        })
    }
}
```

### 4. Add convergence time tests
Verify bounded convergence:

```go
func TestConvergenceTime(t *testing.T) {
    // Test that convergence happens within expected rounds
    // For N clones, should converge in at most N-1 sync rounds

    for n := 3; n <= 10; n++ {
        t.Run(fmt.Sprintf("N=%d", n), func(t *testing.T) {
            rounds := measureConvergenceRounds(t, n)
            maxExpected := n - 1
            if rounds > maxExpected {
                t.Errorf("Convergence took %d rounds, expected ≤ %d",
                    rounds, maxExpected)
            }
        })
    }
}
```

### 5. Add edge case tests
Test boundary conditions:
- All N clones have identical content (dedup works)
- N-1 clones have same content, 1 differs
- All N clones have unique content
- Mix of collisions and non-collisions

## Acceptance Criteria
- TestFiveCloneCollision passes with all sync orders
- All 5 clones converge to identical content
- Performance is acceptable (< 5 seconds for 5 clones)
- Convergence time is bounded (≤ N-1 rounds)
- Edge cases handled correctly
- Benchmarks show scalability to 10+ clones

## Files to Create/Modify
- beads_twoclone_test.go or beads_nway_test.go
- Add helper functions for N-clone setup

## Testing Strategy

### Test Matrix
| N Clones | Sync Orders | Expected Result |
|----------|-------------|-----------------|
| 3        | A→B→C       | Pass            |
| 3        | C→B→A       | Pass            |
| 5        | A→B→C→D→E   | Pass            |
| 5        | E→D→C→B→A   | Pass            |
| 5        | Random      | Pass            |
| 10       | Sequential  | Pass            |

### Performance Targets
- 3 clones: < 2 seconds
- 5 clones: < 5 seconds
- 10 clones: < 15 seconds

## Dependencies
- Requires bd-95, bd-96, bd-97, bd-98 to be completed
- TestThreeCloneCollision must pass first

## Success Metrics
- All tests pass for N ∈ {3, 5, 10}
- Convergence time scales linearly (O(N))
- Memory usage reasonable (< 100MB for 10 clones)
- No data corruption or loss in any scenario