feat: Add metadata.json version tracking validation to bd doctor (bd-u4sb)

Add comprehensive validation of metadata.json version tracking to bd doctor:

Checks added:
- metadata.json exists and is valid JSON
- LastBdVersion field is present and non-empty
- LastBdVersion is valid semver format (e.g., 0.24.2)
- Warns if LastBdVersion is very old (> 10 minor versions behind)
- Provides helpful fix messages for each validation failure

Implementation:
- New checkMetadataVersionTracking() function
- Helper functions: isValidSemver(), parseVersionParts()
- Comprehensive test coverage for all validation scenarios

Tests:
- TestCheckMetadataVersionTracking: 7 test cases covering all scenarios
- TestIsValidSemver: Version format validation
- TestParseVersionParts: Version parsing logic

This helps ensure version tracking (bd-loka) is working correctly and
alerts users if they've missed important upgrade notifications.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.beads/metadata.json

@@ -2,4 +2,4 @@
   "database": "beads.db",
   "jsonl_export": "beads.jsonl",
   "last_bd_version": "0.24.2"
-}
+}