Phase 2 of distributed probing:
- Extract check logic into standalone RunCheck() for use by probes
- Add probe cluster mode: stateless nodes that fetch assignments, execute
checks, and report results to the leader
- Add multi-node result aggregation with configurable strategy
(any-down, majority-down, all-down)
- Leader ingests probe results into engine live state and triggers alerts
- New env vars: UPKEEP_NODE_ID, UPKEEP_NODE_NAME, UPKEEP_NODE_REGION,
UPKEEP_AGG_STRATEGY
- Example docker-compose.probe.yml with leader + 2 regional probes
