Update documentation for hash IDs and aliases

Update all documentation to explain hash-based IDs and aliasing system.

## Files to Update

### 1. README.md
Add section explaining hash IDs:
```markdown
## Issue IDs

Beads uses **hash-based IDs** for collision-free distributed issue tracking:

- **Hash ID**: `bd-af78e9a2` (8-char SHA256 prefix, immutable, globally unique)
- **Alias**: `#42` (sequential number, mutable, human-friendly)

### Using IDs
```bash
bd show bd-af78e9a2   # Use hash ID
bd show #42           # Use alias
bd show 42            # Use alias (shorthand)
```

### Why Hash IDs?
- **Collision-free**: Work offline without ID conflicts
- **Distributed**: No coordination needed between clones
- **Git-friendly**: Different IDs = different JSONL lines, fewer merge conflicts

### Aliases
Aliases are workspace-local shortcuts for hash IDs. They're:
- Automatically assigned on issue creation
- Reassigned deterministically on sync (if conflicts)
- Can be manually controlled with `bd alias` commands
```

### 2. AGENTS.md
Update agent workflow:
```markdown
## Hash-Based IDs (v2.0+)

Beads v2.0 uses hash-based IDs to eliminate collision problems:

**When creating issues**:
```bash
bd create "Fix bug" -p 1
# → Creates bd-af78e9a2 with alias #1
```

**When referencing issues**:
- In text: Use hash IDs (stable): "See bd-af78e9a2 for details"
- In CLI: Use aliases (readable): `bd update #42 --status done`

**After sync**:
- Alias conflicts resolved automatically (content-hash ordering)
- No ID collisions possible
- No remapping needed

**Migration from v1.x**:
```bash
bd migrate --hash-ids  # One-time migration
```
```

### 3. QUICKSTART.md (if exists)
Show alias usage in examples:
```bash
# Create issue (gets hash ID + alias)
bd create "Fix authentication bug" -p 1
# → Created bd-af78e9a2 (alias: #1)

# Reference by alias
bd show #1
bd update #1 --status in_progress
bd close #1 --reason "Fixed"
```

### 4. ADVANCED.md
Add section on hash ID internals:
```markdown
## Hash ID Generation

Hash IDs are generated deterministically:

```go
SHA256(title || description || timestamp || workspace_id)[:8]
```

**Collision probability**:
- 8 hex chars = 2^32 space = ~4 billion IDs
- Birthday paradox: 50% collision probability at ~65,000 issues
- For typical projects (<10,000 issues), collision risk is negligible

**Collision detection**:
If a hash collision occurs (extremely rare), beads:
1. Detects on insert (UNIQUE constraint)
2. Appends random suffix: `bd-af78e9a2-a1b2`
3. Retries insert

## Alias Conflict Resolution

When multiple clones assign same alias to different issues:

**Strategy**: Content-hash ordering (deterministic)
- Sort conflicting issue IDs lexicographically
- Lowest hash ID keeps the alias
- Others reassigned to next available aliases

**Example**:
```
Clone A: Assigns #42 to bd-a1b2c3d4
Clone B: Assigns #42 to bd-e5f6a7b8
After sync: bd-a1b2c3d4 keeps #42 (lower hash)
            bd-e5f6a7b8 gets #100 (next available)
```
```

### 5. MIGRATION.md (new file)
```markdown
# Migrating to Hash-Based IDs (v2.0)

## Overview
Beads v2.0 introduces hash-based IDs to eliminate collision problems. This is a **breaking change** requiring migration.

## Migration Steps

### 1. Backup
```bash
cp -r .beads .beads.backup
git commit -am "Pre-migration backup"
```

### 2. Run Migration
```bash
# Dry run first
bd migrate --hash-ids --dry-run

# Apply migration
bd migrate --hash-ids
```

### 3. Commit Changes
```bash
git add .beads/issues.jsonl
git commit -m "Migrate to hash-based IDs (v2.0)"
git push origin main
```

### 4. Coordinate with Collaborators
All clones must migrate before syncing:
1. Notify team: "Migrating to v2.0 on [date]"
2. All collaborators pull latest
3. All run `bd migrate --hash-ids`
4. All push changes
5. Resume normal work

## Rollback
```bash
# Restore backup
mv .beads.backup .beads
bd export  # Regenerate JSONL
git checkout .beads/issues.jsonl
```

## FAQ

**Q: Can I mix v1.x and v2.0 clones?**
A: No. All clones must be on same version.

**Q: Will my old issue IDs work?**
A: No, but aliases preserve the numbers: bd-1c63eb84 → #1

**Q: What happens to links like "see bd-cb64c226.3"?**
A: Migration updates all text references automatically.
```

### 6. CHANGELOG.md
```markdown
## v2.0.0 (YYYY-MM-DD)

### Breaking Changes
- **Hash-based IDs**: Issues now use collision-free hash IDs (bd-af78e9a2)
  instead of sequential IDs (bd-1c63eb84, bd-9063acda)
- **Aliasing system**: Human-friendly aliases (#42) for hash IDs
- **Migration required**: Run `bd migrate --hash-ids` to convert v1.x databases

### Added
- `bd alias` command for manual alias control
- `bd migrate --hash-ids` migration tool
- Alias conflict resolution (deterministic, content-hash ordering)

### Removed
- ID collision detection and resolution (~2,100 LOC)
- `bd import --resolve-collisions` flag (no longer needed)

### Benefits
- ✅ Zero ID collisions in distributed workflows
- ✅ Simpler codebase (-1,350 net LOC)
- ✅ Better git merge behavior
- ✅ True offline-first operation
```

## Testing
- Build docs locally (if using doc generator)
- Check all links work
- Verify examples are correct
- Spellcheck

## Files to Create/Modify
- README.md (hash ID section)
- AGENTS.md (workflow updates)
- ADVANCED.md (internals)
- MIGRATION.md (new)
- CHANGELOG.md (v2.0 entry)
- docs/ (any other docs)


Extract validation functions to validators.go

Move validatePriority, validateStatus, validateIssueType, validateTitle, validateEstimatedMinutes, validateFieldUpdate to validators.go


Test issue 2


Add migration guide for library consumers

The contributor-workflow-analysis.md has excellent migration examples for CLI users (lines 508-549) but lacks examples for library consumers like VC that use beadsLib in Go/TypeScript code.

Library consumers need to know:
- Whether their existing code continues to work unchanged (backward compatibility)
- How config.toml is automatically read (transparent hydration)
- When and how to use explicit multi-repo configuration
- What happens if config.toml doesn't exist (defaults)

Example needed:
```go
// Before (v0.17.3)
store, err := beadsLib.NewSQLiteStorage(".beads/vc.db")

// After (v0.18.0 with multi-repo) - still works!
store, err := beadsLib.NewSQLiteStorage(".beads/vc.db")
// Automatically reads .beads/config.toml if present

// Explicit multi-repo (if needed)
cfg := beadsLib.Config{
    Primary: ".beads/vc.db",
    Additional: []string{"~/.beads-planning"},
}
store, err := beadsLib.NewStorageWithConfig(cfg)
```