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/theme.yaml

@@ -0,0 +1,20 @@
+apiVersion: theme.halo.run/v1alpha1
+kind: Theme
+metadata:
+  name: theme-vite
+spec:
+  displayName: Vite Theme
+  author:
+    name: Your Name
+    website: https://example.com
+  description: A Halo theme powered by vite-plugin-halo-theme
+  logo: ""
+  homepage: ""
+  repo: ""
+  issues: ""
+  settingName: "theme-vite-setting"
+  configMapName: "theme-vite-configmap"
+  version: 1.0.0
+  requires: ">=2.0.0"
+  license:
+    - name: "MIT"