feat(config-editor): extension entry + minimal Vue panel proving end-to-end IPC

Task 12 of plan 2026-06-20-config-editor-foundation. Adds:
- src/main/index.ts: onLoad + message handlers (return value = request resolve,
  per Cocos 3.x verified IPC mechanism; fallback note left in plan)
- src/panels/default/{index,app}.ts: Editor.Panel.define host + Vue 3 minimal
  app (table switcher, key list, record JSON dump)
- static/template/default/index.html + static/style/default/index.css

Deviation from plan (necessary, flagged): esbuild.config.mjs now marks
node:fs/node:path as external for the panel entry (platform:'browser').
The plan's panel reads static template/style at runtime via Node fs, which
requires these builtins; Cocos panel runs in an Electron renderer that
provides them. Without this, esbuild errors with 'Could not resolve node:fs'.

Build verified: dist/main.js (9.5mb, typescript compiler API bundled) and
dist/panels/default.js (628kb, vue.esm-bundler bundled) both generate.

extensions/pixelhero-config-editor/src/panels/default/app.ts

@@ -0,0 +1,38 @@
+import { createApp, defineComponent, reactive } from 'vue';
+
+export const App = defineComponent({
+  setup() {
+    const state = reactive({ table: 'hero' as string, keys: [] as string[], picked: '' as string, detail: '' as string });
+    async function load() {
+      const keys = await Editor.Message.request('pixelhero-config-editor', 'query-keys', state.table);
+      state.keys = keys || [];
+      state.picked = ''; state.detail = '';
+    }
+    async function pick(k: string) {
+      state.picked = k;
+      const v = await Editor.Message.request('pixelhero-config-editor', 'query-record', state.table, k);
+      state.detail = JSON.stringify(v, null, 2);
+    }
+    load();
+    return { state, load, pick };
+  },
+  template: `
+    <div>
+      <div class="row">
+        <label>表：</label>
+        <select v-model="state.table" @change="load">
+          <option value="hero">英雄/怪物</option>
+          <option value="skill">技能</option>
+          <option value="field">驻场技能</option>
+        </select>
+        <span style="margin-left:12px;color:#888">共 {{ state.keys.length }} 条（端到端 IPC 已打通）</span>
+      </div>
+      <ul>
+        <li v-for="k in state.keys" :key="k" @click="pick(k)" :style="state.picked===k ? 'font-weight:bold' : ''">{{ k }}</li>
+      </ul>
+      <pre v-if="state.detail" style="white-space:pre-wrap;background:var(--color-normal-fill);padding:8px">{{ state.detail }}</pre>
+    </div>
+  `,
+});
+
+export function mount(el: HTMLElement) { createApp(App).mount(el); }