Feat v0.6.0 cluster registry + HA

PG-backed cluster registry for horizontal scaling — zero new
infrastructure (no etcd/Consul/Redis). MVP convergence point.

- node_registry UNLOGGED table (migration 013)
- Self-registration, heartbeat tick (10s), stale sweep (30s),
  self-eviction (os.Exit on 0 rows → K8s restarts)
- GET /api/v1/admin/cluster returns nodes with runtime stats
- Health endpoints include node_id + cluster size/peers
- cluster-dashboard admin surface with auto-refresh
- 3-replica E2E test (docker-compose-e2e.yml + ci/e2e-cluster-test.sh)
- chat + cluster-dashboard added to curated default bundle
- 5 new tests (3 unit + 2 handler), all passing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/store/interfaces.go

@@ -54,6 +54,7 @@ type Stores struct {
 	RateLimits      RateLimitStore           // v0.32.0: Distributed rate limiting
 	Triggers        TriggerStore             // v0.2.2: Extension event/webhook triggers
 	ScheduledTasks  ScheduledTaskStore       // v0.2.2: User-created cron tasks
+	Cluster         ClusterStore             // v0.6.0: PG-backed cluster node registry (nil on SQLite)
 }
 
 // TeamAvailableModel is returned by CatalogStore.ListTeamAvailable.