fix(mcp): allow GET/SSE without session ID for claude.ai compatibility

internal/mcp/server.go

@@ -111,11 +111,8 @@ func (s *Server) handlePOST(w http.ResponseWriter, r *http.Request) {
 }
 
 func (s *Server) handleGET(w http.ResponseWriter, r *http.Request) {
-	sid := r.Header.Get("Mcp-Session-Id")
-	if !s.opts.Sessions.Valid(sid) {
-		http.Error(w, "missing or invalid Mcp-Session-Id", http.StatusBadRequest)
-		return
-	}
+	// Session ID is optional for GET: clients may open the SSE stream before
+	// calling initialize (e.g. claude.ai probes on add). Accept with or without.
 	w.Header().Set("Content-Type", "text/event-stream")
 	w.Header().Set("Cache-Control", "no-cache")
 	w.Header().Set("Connection", "keep-alive")