test(cron): keep script-run tests off the host's PM store

POSIX cron scripts now consult the managed store. Without isolation, a
test run from a PM-managed checkout (the standard ~/.hermes/hermes-agent)
reads the host's store and install records, trips the home I/O guard,
and would run scripts on the host's dependency venv. Point
HERMES_RUNTIME_DIR at an empty per-test dir for tests/cron.

tests/cron/conftest.py

@@ -45,6 +45,13 @@ def make_cron_provider():
     return _make
 
 
+@pytest.fixture(autouse=True)
+def _no_managed_store(tmp_path, monkeypatch):
+    """Point PM's store at an empty dir: script runs must not select the host install's
+    dependency venv (POSIX cron scripts run on it when a store is committed)."""
+    monkeypatch.setenv("HERMES_RUNTIME_DIR", str(tmp_path / "no-pm-store"))
+
+
 @pytest.fixture(autouse=True)
 def _default_cron_test_model(monkeypatch):
     """Pin a default HERMES_MODEL so cron run_job tests have a resolvable model."""