Add rename-prefix command (bd-420)

- Implement bd rename-prefix command with --dry-run and --json flags
- Add prefix validation (max 8 chars, lowercase, starts with letter)
- Update all issue IDs and text references atomically per issue
- Update dependencies, labels, events, and counters
- Fix counter merge to use MAX() to prevent ID collisions
- Update snapshot tables for FK integrity
- Add comprehensive tests for validation and rename workflow
- Document in README.md and AGENTS.md

Known limitation: Each issue updates in its own transaction.
A failure mid-way could leave mixed state. Acceptable for
intended use case (infrequent operation on small DBs).

Amp-Thread-ID: https://ampcode.com/threads/T-7e77b779-bd88-44f2-9f0b-a9f2ccd54d38
Co-authored-by: Amp <amp@ampcode.com>

AGENTS.md

@@ -53,6 +53,10 @@ bd dep tree <id>
 # Get issue details
 bd show <id> --json
 
+# Rename issue prefix (e.g., from 'knowledge-work-' to 'kw-')
+bd rename-prefix kw- --dry-run  # Preview changes
+bd rename-prefix kw- --json     # Apply rename
+
 # Import with collision detection
 bd import -i .beads/issues.jsonl --dry-run             # Preview only
 bd import -i .beads/issues.jsonl --resolve-collisions  # Auto-resolve