CUB-200: implement WebSocket gateway client with v3 protocol

Replace REST poller with WebSocket client as primary gateway connection:

- wsclient.go: WebSocket client with v3 handshake (connect.challenge →
  connect → hello-ok), frame routing (req/res/event), JSON-RPC Send(),
  auto-reconnect with exponential backoff (1s → 30s max)
- sync.go: Initial sync via agents.list + sessions.list RPCs, merge
  session runtime state into AgentCardData, broadcast fleet.update
- events.go: Real-time event handlers for sessions.changed, presence,
  and agent.config — DB update first, then SSE broadcast
- client.go: REST poller retained as fallback (WS is primary)
- config.go: Add GATEWAY_WS_URL and OPENCLAW_GATEWAY_TOKEN env vars
- main.go: Wire WS client as primary, REST as fallback
- .env.example: Document new WS config vars

Fallback: If WS connection fails, seeded demo data + REST polling
remain available.

go-backend/go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/go-chi/cors v1.2.1
 	github.com/go-playground/validator/v10 v10.24.0
 	github.com/google/uuid v1.6.0
+	github.com/gorilla/websocket v1.5.3
 	github.com/jackc/pgx/v5 v5.7.2
 )