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.sum

@@ -17,6 +17,8 @@ github.com/go-playground/validator/v10 v10.24.0 h1:KHQckvo8G6hlWnrPX4NJJ+aBfWNAE
 github.com/go-playground/validator/v10 v10.24.0/go.mod h1:GGzBIJMuE98Ic/kJsBXbz1x/7cByt++cQ+YOuDM5wus=
 github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
 github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/gorilla/websocket v1.5.3 h1:saDtZ6Pbx/0u+bgYQ3q96pZgCzfhKXGPqt7kZ72aNNg=
+github.com/gorilla/websocket v1.5.3/go.mod h1:YR8l580nyteQvAITg2hZ9XVh4b55+EU/adAjf1fMHhE=
 github.com/jackc/pgpassfile v1.0.0 h1:/6Hmqy13Ss2zCq62VdNG8tM1wchn8zjSGOBJ6icpsIM=
 github.com/jackc/pgpassfile v1.0.0/go.mod h1:CEx0iS5ambNFdcRtxPj5JhEz+xB6uRky5eyVu/W2HEg=
 github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 h1:iCEnooe7UlwOQYpKFhBabPMi4aNAfoODPEFNiAnClxo=