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_fix.go

@@ -310,6 +310,8 @@ func applyFixList(path string, fixes []doctorCheck) {
 			// No auto-fix: pruning deletes data, must be user-controlled
 			fmt.Printf("  ⚠ Run 'bd cleanup --older-than 90' to prune old closed issues\n")
 			continue
+		case "Legacy MQ Files":
+			err = doctor.FixStaleMQFiles(path)
 		default:
 			fmt.Printf("  ⚠ No automatic fix available for %s\n", check.Name)
 			fmt.Printf("  Manual fix: %s\n", check.Fix)