feat(cluster): add probe execution mode, check extraction, and result aggregation

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

docker-compose.probe.yml

@@ -0,0 +1,35 @@
+services:
+  leader:
+    build: .
+    environment:
+      - UPKEEP_CLUSTER_MODE=leader
+      - UPKEEP_CLUSTER_SECRET=changeme
+      - UPKEEP_AGG_STRATEGY=any-down
+      - UPKEEP_STATUS_ENABLED=true
+    ports:
+      - "8080:8080"
+      - "23234:23234"
+
+  probe-us-east:
+    build: .
+    environment:
+      - UPKEEP_CLUSTER_MODE=probe
+      - UPKEEP_NODE_ID=us-east-1
+      - UPKEEP_NODE_NAME=US East Probe
+      - UPKEEP_NODE_REGION=us-east
+      - UPKEEP_PEER_URL=http://leader:8080
+      - UPKEEP_CLUSTER_SECRET=changeme
+    depends_on:
+      - leader
+
+  probe-eu-west:
+    build: .
+    environment:
+      - UPKEEP_CLUSTER_MODE=probe
+      - UPKEEP_NODE_ID=eu-west-1
+      - UPKEEP_NODE_NAME=EU West Probe
+      - UPKEEP_NODE_REGION=eu-west
+      - UPKEEP_PEER_URL=http://leader:8080
+      - UPKEEP_CLUSTER_SECRET=changeme
+    depends_on:
+      - leader