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>

src/main/java/run/halo/darkmode/DarkModePlugin.java

@@ -0,0 +1,31 @@
+package run.halo.darkmode;
+
+import org.springframework.stereotype.Component;
+import run.halo.app.plugin.BasePlugin;
+import run.halo.app.plugin.PluginContext;
+
+/**
+ * <p>Plugin main class to manage the lifecycle of the plugin.</p>
+ * <p>This class must be public and have a public constructor.</p>
+ * <p>Only one main class extending {@link BasePlugin} is allowed per plugin.</p>
+ *
+ * @author LHY
+ * @since 1.0.0
+ */
+@Component
+public class DarkModePlugin extends BasePlugin {
+
+    public DarkModePlugin(PluginContext pluginContext) {
+        super(pluginContext);
+    }
+
+    @Override
+    public void start() {
+        System.out.println("插件启动成功！");
+    }
+
+    @Override
+    public void stop() {
+        System.out.println("插件停止！");
+    }
+}