feat(doctor): add check for stale .beads/mq/ files (bd-dx2dc)

Add bd doctor check that detects legacy gastown merge queue JSON files
in .beads/mq/. These files are local-only remnants from the old mrqueue
implementation and can safely be deleted since gt done already creates
merge-request wisps in beads.

- CheckStaleMQFiles() detects .beads/mq/*.json files
- FixStaleMQFiles() removes the entire mq directory
- Comprehensive tests for check and fix

This is the first step toward removing the mrqueue side-channel from
gastown. The follow-up convoy will update Refinery/Witness to use
beads exclusively.

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

cmd/bd/doctor.go

@@ -572,6 +572,11 @@ func runDiagnostics(path string) doctorResult {
 	result.Checks = append(result.Checks, persistentMolCheck)
 	// Don't fail overall check for persistent mol issues, just warn
 
+	// Check 26c: Legacy merge queue files (gastown mrqueue remnants)
+	staleMQFilesCheck := convertDoctorCheck(doctor.CheckStaleMQFiles(path))
+	result.Checks = append(result.Checks, staleMQFilesCheck)
+	// Don't fail overall check for legacy MQ files, just warn
+
 	// Check 27: Expired tombstones (maintenance)
 	tombstonesExpiredCheck := convertDoctorCheck(doctor.CheckExpiredTombstones(path))
 	result.Checks = append(result.Checks, tombstonesExpiredCheck)