Add heartbeat job: 5-min DM health check\n\nSystemd timer job-heartbeat.timer runs every 5 minutes,\ndispatches heartbeat DM to opm via job-dispatch.py.

jobs/heartbeat.json

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+{"agent":"opm","chain_next":null,"description":"Heartbeat job - verifies DM system is working every 5 minutes","name":"heartbeat","on_failure":"alert","prompt_template":"Heartbeat check from job scheduler.\nJob ID: {job_id}\nTime: {datetime}\n\nThis is an automated heartbeat. Reply with [RESULT {job_id}] OK to confirm the DM pipeline is healthy.","schedule":"*/5 * * * *","sidechat":{"create":false},"timeout":300}