fix(cron): add concurrency regression test for parallel job state writes

get_due_jobs() called load_jobs() and save_jobs() without holding
_jobs_file_lock, creating a race with the locked mark_job_run() and
advance_next_run(). Wrap get_due_jobs() with the lock (delegating to a
new _get_due_jobs_locked() inner function) so all load→modify→save
cycles are serialised. Add two regression tests: one verifying 3
concurrent mark_job_run() calls each land their correct last_status and
last_run_at without overwrites, and a stress test confirming 10 parallel
calls each increment their job's completed count to exactly 1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cron/jobs.py

@@ -810,6 +810,12 @@ def get_due_jobs() -> List[Dict[str, Any]]:
     the job is fast-forwarded to the next future run instead of firing
     immediately.  This prevents a burst of missed jobs on gateway restart.
     """
+    with _jobs_file_lock:
+        return _get_due_jobs_locked()
+
+
+def _get_due_jobs_locked() -> List[Dict[str, Any]]:
+    """Inner implementation of get_due_jobs(); must be called with _jobs_file_lock held."""
     now = _hermes_now()
     raw_jobs = load_jobs()
     jobs = [_apply_skill_fields(j) for j in copy.deepcopy(raw_jobs)]