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