fix(cron): retire persisted heartbeat/dream jobs when disabled

Disabling gateway.heartbeat (or agents.defaults.dream) only skipped
job registration on startup; the previously persisted system job in
<workspace>/cron/jobs.json kept firing, and remove_job refuses to
touch protected system jobs. Add CronService.remove_system_job for
startup reconciliation and call it from the gateway disabled branches
so the config toggle takes effect after restart.

tests/cli/test_commands.py

@@ -3221,6 +3221,9 @@ def test_gateway_local_trigger_queue_submits_agent_turns(
         def register_system_job(self, _job) -> None:
             return None
 
+        def remove_system_job(self, _job_id: str) -> bool:
+            return False
+
     class _FakeAgentLoop(_GatewayAgentContractStub):
         @classmethod
         def from_config(cls, config, bus=None, **extra):