add official Chrome feature demos

public/demo/devtools-agents/app.js

@@ -0,0 +1,14 @@
+console.warn("Demo warning: this page intentionally includes diagnosable issues.");
+
+const longTaskButton = document.querySelector("#long-task");
+const taskResult = document.querySelector("#task-result");
+
+longTaskButton.addEventListener("click", () => {
+  const start = performance.now();
+  while (performance.now() - start < 650) {
+    Math.sqrt(Math.random() * Number.MAX_SAFE_INTEGER);
+  }
+  const duration = Math.round(performance.now() - start);
+  taskResult.textContent = `已触发约 ${duration}ms 的主线程长任务。`;
+  console.error("Demo error: long task was triggered for DevTools MCP inspection.");
+});