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.
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