feat(docker): add /healthz endpoint and -healthcheck flag (#254)

Adds a Docker healthcheck and the small HTTP/CLI plumbing it needs.

- Expose `GET /healthz` on the HTTP-mode server.
- Add `-healthcheck` to dial `http://127.0.0.1:<port>/healthz` and exit with status.
- Add a Dockerfile `HEALTHCHECK` instruction using `-healthcheck`.
- Document HTTP-mode usage and stdio override guidance.

Closes https://gitea.com/gitea/gitea-mcp/issues/146

Assisted by Codet.

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/254
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

operation/operation.go

@@ -133,6 +133,7 @@ func newHTTPServer(addr string, s *mcp.Server) *http.Server {
 			PropagateRequestCancellation: true,
 		},
 	)))
+	mux.HandleFunc("/healthz", handleHealthz)
 	return &http.Server{
 		Addr:              addr,
 		Handler:           mux,
@@ -140,6 +141,12 @@ func newHTTPServer(addr string, s *mcp.Server) *http.Server {
 	}
 }
 
+func handleHealthz(w http.ResponseWriter, _ *http.Request) {
+	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte("ok\n"))
+}
+
 func Run() error {
 	mcpServer = newMCPServer(flag.Version)
 	RegisterTool(mcpServer)