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>

skills-lock.json

@@ -0,0 +1,17 @@
+{
+  "version": 1,
+  "skills": {
+    "halo-plugin-dev": {
+      "source": "halo-dev/dev-skills",
+      "sourceType": "github",
+      "skillPath": "skills/halo-plugin-dev/SKILL.md",
+      "computedHash": "cacdbde0d53b6d062b3acbe237c6dbe60fd564d7bb70172b22eb4cfbf43117dd"
+    },
+    "halo-theme-dev": {
+      "source": "halo-dev/dev-skills",
+      "sourceType": "github",
+      "skillPath": "skills/halo-theme-dev/SKILL.md",
+      "computedHash": "19045f67d7f115d68fdba5704ef332606caf35f67f1be3ba0c7b20fea7704767"
+    }
+  }
+}