feat(cron): add deploy sync guard to catch stale code before cascading failures

When the installed run_agent.py diverges from what scheduler.py expects,
every cron job fails with TypeError on AIAgent.__init__() — a silent total
outage that cascades into gateway restarts, asyncio shutdown errors, and
auth token expiry.

This commit adds a _validate_agent_interface() guard that:
- Inspects AIAgent.__init__ at runtime via inspect.signature
- Verifies every kwarg the scheduler passes exists in the constructor
- Fails fast with a clear remediation message on mismatch
- Runs once per gateway process (cached, zero per-job overhead)

The guard is called at the top of run_job() before any work begins.
It would have caught the tool_choice TypeError that caused 1,199 failures
across 55 jobs (meta-issue #343).

Includes 3 tests: pass, fail, and cache verification.

tests/cron/test_scheduler.py

@@ -863,3 +863,54 @@ class TestTickAdvanceBeforeRun:
         adv_mock.assert_called_once_with("test-advance")
         # advance must happen before run
         assert call_order == [("advance", "test-advance"), ("run", "test-advance")]
+
+
+class TestDeploySyncGuard:
+    """Tests for _validate_agent_interface() — the deploy sync guard."""
+
+    def test_passes_when_all_params_present(self):
+        """Validation passes when AIAgent accepts every scheduler kwarg."""
+        from cron.scheduler import _validate_agent_interface, _agent_interface_validated
+        import cron.scheduler as sched_mod
+
+        # Reset the cached flag so the check actually runs.
+        sched_mod._agent_interface_validated = False
+        # Should not raise.
+        _validate_agent_interface()
+        assert sched_mod._agent_interface_validated is True
+
+    def test_fails_when_param_missing(self):
+        """Validation raises RuntimeError when AIAgent is missing a required param."""
+        import cron.scheduler as sched_mod
+        from unittest.mock import MagicMock
+        import inspect
+
+        # Save and restore.
+        orig_flag = sched_mod._agent_interface_validated
+        try:
+            sched_mod._agent_interface_validated = False
+
+            # Build a fake AIAgent class whose __init__ lacks 'tool_choice'.
+            class FakeAIAgent:
+                def __init__(self, model="", max_iterations=90, quiet_mode=False,
+                             disabled_toolsets=None, skip_memory=False, platform=None,
+                             session_id=None, session_db=None):
+                    pass
+
+            fake_module = MagicMock()
+            fake_module.AIAgent = FakeAIAgent
+
+            with pytest.raises(RuntimeError, match="Missing parameters: tool_choice"):
+                with patch.dict("sys.modules", {"run_agent": fake_module}):
+                    sched_mod._validate_agent_interface()
+        finally:
+            sched_mod._agent_interface_validated = orig_flag
+
+    def test_cached_after_first_run(self):
+        """Second call is a no-op (uses cached flag)."""
+        import cron.scheduler as sched_mod
+
+        sched_mod._agent_interface_validated = True
+        # Should not raise even if we somehow break AIAgent — the flag is set.
+        sched_mod._validate_agent_interface()
+        # No exception = pass.