feat: 添加 /plugin 插件市场命令

功能:
- /plugin list - 列出可用插件（自动同步远程源）
- /plugin search <关键词> - 搜索插件
- /plugin info <插件码> - 查看插件详情
- /plugin install <插件码|URL> - 安装插件（自动下载、启用、动态加载）
- /plugin remove <名称> - 卸载插件
- /plugin update <插件码> - 更新插件
- /plugin add-source <URL> - 添加远程插件源

文件:
- xcli_core/plugin_market.py - PluginMarketMixin 核心逻辑
- plugin_registry.json - 本地插件码注册表
- xcli_core/cli_core.py - 集成 PluginMarketMixin
- HELP.md - 添加插件市场文档

plugin_registry.json

@@ -0,0 +1,18 @@
+{
+  "version": 1,
+  "description": "小狸 CLI 插件码注册表 - 通过 /plugin install <插件码> 安装",
+  "plugins": {
+    "dglab": {
+      "name": "DG-Lab 郊狼 WebSocket 控制",
+      "description": "基于 DG-LAB-OPENSOURCE 官方 Socket V2 协议，支持郊狼 3.0 脉冲主机远程控制",
+      "author": "xiaoli-cli-plugins",
+      "version": "1.0.0",
+      "file": "dglab_ws.py",
+      "url": "https://gitee.com/shuiyu1123/xiaoli-cli-plugins/raw/master/dglab_ws/dglab_ws.py",
+      "docs": "https://gitee.com/shuiyu1123/xiaoli-cli-plugins/raw/master/dglab_ws/README.md",
+      "deps": ["websocket-client"],
+      "optional_deps": ["qrcode"],
+      "keywords": ["dg-lab", "dglab", "郊狼", "coyote", "websocket", "脉冲"]
+    }
+  }
+}