Add /version endpoint with build metadata

- /version returns JSON with build_date and git_commit from env vars
- NixOS module injects GIT_COMMIT (first 8 chars of self.rev)
  and BUILD_DATE (self.lastModified) into systemd service environment

main.go

@@ -83,6 +83,13 @@ func main() {
 		w.WriteHeader(http.StatusOK)
 		w.Write([]byte("ok"))
 	})
+	mux.HandleFunc("/version", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(map[string]string{
+			"build_date": os.Getenv("BUILD_DATE"),
+			"git_commit": os.Getenv("GIT_COMMIT"),
+		})
+	})
 	mux.Handle("/mcp", authMiddleware(token, streamableServer))
 
 	httpServer := &http.Server{