feat(agent): add WebSocket transport with HTTP fallback

Add Transport interface abstraction supporting WebSocket (via CF
Durable Objects) and HTTP (direct to origin) with automatic failover.

- Transport interface: Register, SendHeartbeat, SendProgress, Events()
- HTTPTransport: thin adapter over existing Client
- WSTransport: gorilla/websocket with auth handshake, readLoop, reconnect
- HybridTransport: tries WS first, falls back to HTTP, reconnects in bg
- Daemon refactored to always use Transport (no dual-path forks)
- ProgressReporter accepts StatusReporter interface
- deriveWSURL skips localhost/dev (returns "" → HTTP-only)
- API key passed in WS query param for connection auth
- Fixed: reconnectOnce race (mutex+bool), authDone double-close (sync.Once)
- Fixed: forwardWSEvents goroutine leak (select with stop signal)
- 20 transport tests + 2 E2E tests (full lifecycle, hybrid failover)

internal/config/config.go

@@ -25,6 +25,7 @@ type Config struct {
 type AuthConfig struct {
 	APIKey string `toml:"api_key"`
 	APIURL string `toml:"api_url"`
+	WSURL  string `toml:"ws_url"` // optional, derived from api_url if empty
 }
 
 type AgentConfig struct {