feat: Add bd repair command for orphaned foreign key refs (hq-2cchm)

When the database has orphaned dependencies or labels, the migration
invariant check fails and prevents the database from opening. This
creates a chicken-and-egg problem where bd doctor --fix cannot run.

The new bd repair command:
- Opens SQLite directly, bypassing invariant checks
- Deletes orphaned dependencies (issue_id or depends_on_id not in issues)
- Deletes orphaned labels (issue_id not in issues)
- Runs WAL checkpoint to persist changes
- Supports --dry-run to preview what would be cleaned

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/main.go

@@ -281,6 +281,7 @@ var rootCmd = &cobra.Command{
 			"powershell",
 			"prime",
 			"quickstart",
+			"repair",
 			"setup",
 			"version",
 			"zsh",