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>

tests/cron/test_jobs.py

@@ -1,6 +1,7 @@
 """Tests for cron/jobs.py — schedule parsing, job CRUD, and due-job detection."""
 
 import json
+import threading
 import pytest
 from datetime import datetime, timedelta, timezone
 from pathlib import Path
@@ -745,6 +746,100 @@ class TestEnabledToolsets:
         assert fetched["enabled_toolsets"] == ["web", "delegation"]
 
 
+class TestMarkJobRunConcurrency:
+    """Regression tests for concurrent parallel job state writes.
+
+    tick() dispatches multiple jobs to separate threads simultaneously.
+    Without _jobs_file_lock protecting the load→modify→save cycle in
+    mark_job_run(), concurrent writes can clobber each other's updates
+    (last-writer-wins), leaving some jobs with stale last_status / last_run_at.
+    """
+
+    def test_three_concurrent_mark_job_run_no_overwrites(self, tmp_cron_dir):
+        """Run mark_job_run() for 3 jobs in parallel threads; all must land correctly."""
+        # Create 3 distinct recurring jobs
+        job_a = create_job(prompt="Job A", schedule="every 1h")
+        job_b = create_job(prompt="Job B", schedule="every 1h")
+        job_c = create_job(prompt="Job C", schedule="every 1h")
+
+        errors: list = []
+
+        def run_mark(job_id: str, success: bool, error_msg=None):
+            try:
+                mark_job_run(job_id, success=success, error=error_msg)
+            except Exception as exc:  # pragma: no cover
+                errors.append(exc)
+
+        # Fire all three concurrently
+        threads = [
+            threading.Thread(target=run_mark, args=(job_a["id"], True)),
+            threading.Thread(target=run_mark, args=(job_b["id"], False, "timeout")),
+            threading.Thread(target=run_mark, args=(job_c["id"], True)),
+        ]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        assert not errors, f"Unexpected exceptions in worker threads: {errors}"
+
+        # Verify each job has the correct state — no overwrites
+        a = get_job(job_a["id"])
+        b = get_job(job_b["id"])
+        c = get_job(job_c["id"])
+
+        assert a is not None, "Job A was unexpectedly deleted"
+        assert b is not None, "Job B was unexpectedly deleted"
+        assert c is not None, "Job C was unexpectedly deleted"
+
+        assert a["last_status"] == "ok", f"Job A last_status wrong: {a['last_status']}"
+        assert a["last_run_at"] is not None, "Job A last_run_at not set"
+        assert a["repeat"]["completed"] == 1, f"Job A completed count wrong: {a['repeat']['completed']}"
+
+        assert b["last_status"] == "error", f"Job B last_status wrong: {b['last_status']}"
+        assert b["last_error"] == "timeout", f"Job B last_error wrong: {b['last_error']}"
+        assert b["last_run_at"] is not None, "Job B last_run_at not set"
+        assert b["repeat"]["completed"] == 1, f"Job B completed count wrong: {b['repeat']['completed']}"
+
+        assert c["last_status"] == "ok", f"Job C last_status wrong: {c['last_status']}"
+        assert c["last_run_at"] is not None, "Job C last_run_at not set"
+        assert c["repeat"]["completed"] == 1, f"Job C completed count wrong: {c['repeat']['completed']}"
+
+    def test_repeated_concurrent_runs_accumulate_completed_count(self, tmp_cron_dir):
+        """Stress test: 10 threads each call mark_job_run on a different job once.
+
+        The completed count for every job must be exactly 1 after all threads finish,
+        confirming no thread's write was silently dropped.
+        """
+        n = 10
+        jobs = [create_job(prompt=f"Stress job {i}", schedule="every 1h") for i in range(n)]
+        errors: list = []
+
+        def run_mark(job_id: str):
+            try:
+                mark_job_run(job_id, success=True)
+            except Exception as exc:  # pragma: no cover
+                errors.append(exc)
+
+        threads = [threading.Thread(target=run_mark, args=(j["id"],)) for j in jobs]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        assert not errors, f"Unexpected exceptions: {errors}"
+
+        for job in jobs:
+            updated = get_job(job["id"])
+            assert updated is not None, f"Job {job['id']} was deleted"
+            assert updated["last_status"] == "ok", (
+                f"Job {job['id']} has wrong last_status: {updated['last_status']}"
+            )
+            assert updated["repeat"]["completed"] == 1, (
+                f"Job {job['id']} completed count is {updated['repeat']['completed']}, expected 1"
+            )
+
+
 class TestSaveJobOutput:
     def test_creates_output_file(self, tmp_cron_dir):
         output_file = save_job_output("test123", "# Results\nEverything ok.")