CUB-201: add initial sync via agents.list + sessions.list RPCs

- Create gateway/sync.go with initialSync method on WSClient
- Fetch agents via agents.list RPC, persist to AgentRepo
- Fetch sessions via sessions.list RPC, map status to AgentStatus
- Merge session state (status, sessionKey, tokens) into AgentCardData
- Broadcast merged fleet as fleet.update via SSE broker
- Trigger initialSync after hello-ok handshake
- Re-sync automatically on reconnect (connectAndRun calls initialSync)
- Handle unknown gateway fields gracefully via typed extraction

go-backend/internal/gateway/wsclient.go

@@ -53,6 +53,7 @@ type WSClient struct {
 	logger  *slog.Logger
 
 	handlers map[string][]eventHandler
+	connId   string // set after successful hello-ok
 }
 
 // NewWSClient returns a WSClient wired to the given repository and broker.
@@ -202,6 +203,16 @@ func (c *WSClient) connectAndRun(ctx context.Context) error {
 		"methods", helloOK.Features.Methods,
 		"events", helloOK.Features.Events)
 
+	// Store connId for reference
+	c.connMu.Lock()
+	c.connId = helloOK.ConnID
+	c.connMu.Unlock()
+
+	// Step 2b: Initial sync — fetch agents + sessions from gateway
+	if err := c.initialSync(ctx); err != nil {
+		c.logger.Warn("initial sync failed, will continue with read loop", "error", err)
+	}
+
 	// Step 3: Read loop
 	return c.readLoop(ctx, conn)
 }