fix(mcp): add SSE GET handler for streamable HTTP transport

claude.ai probes with GET before initialize; without this the supervisor
returned application/json parse error instead of text/event-stream, causing
"Couldn't reach the MCP server" in the claude.ai connector setup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/mcp/server.go

@@ -48,6 +48,22 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// GET opens the SSE stream for server-to-client events (MCP streamable HTTP).
+	// claude.ai probes with GET before sending initialize, so accept without a session.
+	if r.Method == http.MethodGet {
+		w.Header().Set("Content-Type", "text/event-stream")
+		w.Header().Set("Cache-Control", "no-cache")
+		w.Header().Set("Connection", "keep-alive")
+		w.Header().Set("X-Accel-Buffering", "no")
+		w.WriteHeader(http.StatusOK)
+		if f, ok := w.(http.Flusher); ok {
+			_, _ = w.Write([]byte(": stream open\n\n"))
+			f.Flush()
+		}
+		<-r.Context().Done()
+		return
+	}
+
 	var req request
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeError(w, nil, -32700, "parse error")