fix: complete cron jobs lock salvage

Route curator rollback through the same cross-process cron job lock, make save_jobs lock for legacy direct callers without deadlocking nested mutation paths, and harden the regression test so a second _jobs_lock caller really blocks across processes.

agent/curator_backup.py

@@ -454,16 +454,16 @@ def _restore_cron_skill_links(snapshot_dir: Path) -> Dict[str, Any]:
         report["attempted"] = True  # we tried but there was nothing to do
         return report
 
-    # Load and rewrite the live jobs under the scheduler's lock.
+    # Load and rewrite the live jobs under the scheduler's cross-process lock.
     try:
-        from cron.jobs import load_jobs, save_jobs, _jobs_file_lock
+        from cron.jobs import load_jobs, save_jobs, _jobs_lock
     except ImportError as e:
         report["error"] = f"cron module unavailable: {e}"
         return report
 
     report["attempted"] = True
     try:
-        with _jobs_file_lock:
+        with _jobs_lock():
             live_jobs = load_jobs()
             changed = False