feat: Add stale molecules check to bd doctor (bd-6a5z)

Extends bd doctor to detect complete-but-unclosed molecules (epics where
all children are closed but root is still open).

- Added CheckStaleMolecules() to doctor/maintenance.go
- Added resolveBeadsDir() helper to follow Gas Town redirect files
- Check appears in Maintenance category with warning severity
- Shows example IDs and suggests 'bd mol stale' for review

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

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

cmd/bd/doctor.go

@@ -814,6 +814,11 @@ func runDiagnostics(path string) doctorResult {
 	result.Checks = append(result.Checks, staleClosedCheck)
 	// Don't fail overall check for stale issues, just warn
 
+	// Check 26a: Stale molecules (complete but unclosed, bd-6a5z)
+	staleMoleculesCheck := convertDoctorCheck(doctor.CheckStaleMolecules(path))
+	result.Checks = append(result.Checks, staleMoleculesCheck)
+	// Don't fail overall check for stale molecules, just warn
+
 	// Check 27: Expired tombstones (maintenance, bd-bqcc)
 	tombstonesExpiredCheck := convertDoctorCheck(doctor.CheckExpiredTombstones(path))
 	result.Checks = append(result.Checks, tombstonesExpiredCheck)