feat: 初始化 Halo 暗色模式插件

- Halo Plugin 后端（Java/Gradle），含 DarkModePlugin 主类和测试
- Vue 3 + TypeScript 前端 UI，包含主题切换组件和设置页面
- 暗色模式 CSS 变量和覆盖样式（布局/编辑器/表单/滚动条等）
- 设计文档和调查文档
- Halo 插件/主题开发 Agent Skills

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

.agents/skills/halo-theme-dev/assets/theme-vite/src/author.html

@@ -0,0 +1,16 @@
+<include src="layout.html">
+  <template name="head">
+    <title th:text="|${author.spec.displayName} - ${site.title}|">Author</title>
+  </template>
+
+  <section>
+    <h1 th:text="${author.spec.displayName}"></h1>
+    <p th:if="${not #strings.isEmpty(author.spec.bio)}" th:text="${author.spec.bio}"></p>
+
+    <ul class="post-list">
+      <include src="post-card.html"></include>
+    </ul>
+
+    <include src="pagination.html"></include>
+  </section>
+</include>