Add bd migrate command for database upgrades (bd-164)

- Implement bd migrate command with detection, version checking, and cleanup
- Update daemon to suggest bd migrate for version mismatches
- Enhance CLI version warnings to recommend bd migrate
- Add comprehensive tests for migration scenarios
- Document migration workflow in QUICKSTART.md and AGENTS.md

Completes bd-164 and epic bd-159

Amp-Thread-ID: https://ampcode.com/threads/T-34ea4682-8c48-44c2-8421-dc40f867773b
Co-authored-by: Amp <amp@ampcode.com>

AGENTS.md

@@ -198,6 +198,11 @@ bd duplicates --dry-run                                # Preview merge operation
 # Merge specific duplicate issues
 bd merge <source-id...> --into <target-id> --json      # Consolidate duplicates
 bd merge bd-42 bd-43 --into bd-41 --dry-run            # Preview merge
+
+# Migrate databases after version upgrade
+bd migrate                                             # Detect and migrate old databases
+bd migrate --dry-run                                   # Preview migration
+bd migrate --cleanup --yes                             # Migrate and remove old files
 ```
 
 ### Managing Daemons