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>

CHANGELOG.md

@@ -2,6 +2,44 @@
 
 All notable changes to Switchboard Core are documented here.
 
+## v0.6.0 — Cluster Registry + HA
+
+MVP convergence point. PG-backed cluster registry for horizontal scaling —
+zero new infrastructure (no etcd/Consul/Redis).
+
+### Added
+
+- **Cluster registry**: `node_registry` UNLOGGED table with self-registration
+  on startup (`INSERT ... ON CONFLICT DO UPDATE`), heartbeat tick (default 10s),
+  stale sweep (default 30s threshold), and self-eviction (`os.Exit(1)` when
+  swept by a peer — K8s restarts the process).
+- **Cluster admin API**: `GET /api/v1/admin/cluster` returns all registered
+  nodes with runtime stats (goroutines, heap, GC, uptime, ws_clients) in the
+  standard `{data: [...]}` envelope.
+- **Health endpoint cluster info**: `GET /health` and `GET /api/v1/health`
+  now include `node_id` and `cluster: {size, peers, heartbeat_age_ms}` when
+  running on Postgres with the registry active.
+- **Cluster dashboard**: `cluster-dashboard` admin surface package renders one
+  card per node with live runtime stats. Auto-refreshes every 10s. Stats are
+  JSONB — new keys render automatically without schema migration.
+- **Cluster config**: `CLUSTER_NODE_ID` (default hostname-PID),
+  `CLUSTER_HEARTBEAT_INTERVAL` (default 10s), `CLUSTER_STALE_THRESHOLD`
+  (default 30s), `CLUSTER_ENDPOINT` (Phase 2 mesh, auto-detect).
+- **3-replica E2E test**: `docker-compose-e2e.yml` now runs 3 replicas.
+  `ci/e2e-cluster-test.sh` verifies registration, stale sweep on stop, and
+  re-registration on restart.
+- **5 new tests**: 3 cluster registry unit tests (stats collection, start/stop
+  lifecycle, node ID) + 2 handler tests (list nodes, empty response).
+- **Curated bundle expanded**: `chat` and `cluster-dashboard` added to the
+  default bundle (now 10 packages). Production deployments can override with
+  `BUNDLED_PACKAGES=notes,chat,chat-core,cluster-dashboard` for a lean set.
+
+### Changed
+
+- SQLite deployments are unaffected — cluster store is nil, all cluster code
+  is guarded behind `database.IsPostgres() && stores.Cluster != nil`.
+  Cluster dashboard gracefully shows "0 nodes registered" on SQLite.
+
 ## v0.5.5 — Upgrade Testing
 
 ### Fixed