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！");
+    }
+}

src/main/resources/plugin.yaml

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+# Refer https://docs.halo.run/developer-guide/plugin/basics/manifest
+
+apiVersion: plugin.halo.run/v1alpha1
+kind: Plugin
+metadata:
+  # The name defines how the plugin is invoked, A unique name
+  name: dark-mode
+spec:
+  enabled: true
+  requires: ">=2.25.0"
+  author:
+    name: LHY
+    website: https://github.com/LHY
+  logo: logo.png
+  homepage: https://github.com/LHY/dark-mode#readme
+  repo: https://github.com/LHY/dark-mode
+  issues: https://github.com/LHY/dark-mode/issues
+  displayName: "深色模式"
+  description: "为 Halo 后台管理面板提供深色/浅色模式切换，支持跟随系统、手动切换和偏好记忆"
+  license:
+    - name: "GPL-3.0"
+      url: "https://github.com/LHY/dark-mode/blob/main/LICENSE"

src/test/java/run/halo/darkmode/DarkModePluginTest.java

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+package run.halo.darkmode;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import run.halo.app.plugin.PluginContext;
+
+@ExtendWith(MockitoExtension.class)
+class DarkModePluginTest {
+
+    @Mock
+    PluginContext context;
+
+    @InjectMocks
+    DarkModePlugin plugin;
+
+    @Test
+    void contextLoads() {
+        plugin.start();
+        plugin.stop();
+    }
+}