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.

.env.example

@@ -12,8 +12,14 @@ ENVIRONMENT=development
 # Format: postgresql://user:password@host:port/database?sslmode=disable
 DATABASE_URL=postgresql://controlcenter:controlcenter@localhost:5432/controlcenter?sslmode=disable
 
-# Gateway (OpenClaw) connection
-# URL to the OpenClaw gateway API for polling agent states
+# Gateway (OpenClaw) connection — WebSocket (primary)
+# WebSocket URL for real-time OpenClaw gateway v3 protocol
+GATEWAY_WS_URL=ws://localhost:18789/
+# Auth token for the OpenClaw gateway (operator scope)
+OPENCLAW_GATEWAY_TOKEN=
+
+# Gateway (OpenClaw) connection — REST (fallback)
+# URL to the OpenClaw gateway API for polling agent states (used only if WS fails)
 GATEWAY_URL=http://localhost:18789/api/agents
 # Polling interval for agent state updates
 GATEWAY_POLL_INTERVAL=5s