fix(cron): reject id mutation + validate output paths under OUTPUT_DIR

Two defense-in-depth fixes on cron output path handling:

1. cron/jobs.py:update_job() rejects mutation of the immutable 'id' field
   (raises ValueError). Dashboard PUT /api/cron/jobs/{id} converts this to
   HTTP 400. Without this, an attacker who can reach the update endpoint
   could rename a job's id to '../escape' and move its output directory
   outside OUTPUT_DIR.

2. cron/jobs.py:_job_output_dir() validates job IDs before composing
   paths: rejects '.', '..', '/', '\\', absolute paths, and Windows drive
   prefixes. Used by save_job_output() and remove_job() so legacy unsafe
   IDs (from before this guard) fail closed rather than half-applying a
   shutil.rmtree or output write outside the sandbox.

Tests:
  - update_job rejects {'id': '../escape'} without renaming
  - remove_job(legacy '../escape' id) raises ValueError without deleting
    files outside OUTPUT_DIR or removing the job from the store
  - save_job_output rejects '..', './escape', 'nested/escape',
    absolute paths
  - dashboard PUT /api/cron/jobs/{id} with {'id': '../escape'} returns
    400, job list unchanged

Salvaged from PR #29826 by @zapabob. Simplified implementation:
- Dropped a 23-line _validate_job_output_id() helper using Path.parts
  semantics. The inline check (path separators + dot-components +
  is_absolute) is shorter and behaviorally identical.
- Dropped the secondary OUTPUT_DIR.resolve()/relative_to() check —
  redundant once we reject any path separator at the input boundary.
- Dropped the _docs/2026-05-21_cron-output-path-hardening_codex.md
  planning artifact (we don't check planning docs into the repo).

Co-authored-by: teknium1 <127238744+teknium1@users.noreply.github.com>

tests/hermes_cli/test_web_server_cron_profiles.py

@@ -131,6 +131,33 @@ async def test_cron_mutation_without_profile_finds_named_profile_job(isolated_pr
     assert worker_jobs[0]["enabled"] is False
 
 
+@pytest.mark.asyncio
+async def test_update_cron_job_rejects_id_mutation(isolated_profiles):
+    """Dashboard surfaces a 400 (not a 500 or silent rename) when an
+    id-mutation attempt is rejected by cron/jobs.update_job."""
+    from hermes_cli import web_server
+
+    worker_job = web_server._call_cron_for_profile(
+        "worker_alpha",
+        "create_job",
+        prompt="managed by named profile",
+        schedule="every 1h",
+        name="immutable-id-job",
+    )
+
+    with pytest.raises(HTTPException) as exc:
+        await web_server.update_cron_job(
+            worker_job["id"],
+            web_server.CronJobUpdate(updates={"id": "../escape"}),
+            profile="worker_alpha",
+        )
+
+    assert exc.value.status_code == 400
+    assert "id" in exc.value.detail
+    worker_jobs = await web_server.list_cron_jobs(profile="worker_alpha")
+    assert [job["id"] for job in worker_jobs] == [worker_job["id"]]
+
+
 @pytest.mark.asyncio
 async def test_cron_delete_with_profile_deletes_only_target_profile(isolated_profiles):
     from hermes_cli import web_server