feat: 添加 Git 工具集和重组描述文件目录结构

Git 工具集:
- 新增 10 个 Git 工具: status, diff, log, branch, add, commit, push, pull, checkout, stash
- 新增 GitPermissionChecker 权限检查器
- 读操作默认允许，写操作需要用户确认

目录结构优化:
- 重组 descriptions 目录，按工具类别分子目录 (shell, filesystem, web, git, todo)
- 更新 load_description.ts 支持子目录加载
- 更新 build 脚本支持递归复制描述文件

package.json

@@ -8,7 +8,7 @@
   },
   "type": "module",
   "scripts": {
-    "build": "tsc && cp -r src/tools/descriptions/*.txt dist/tools/descriptions/",
+    "build": "tsc && mkdir -p dist/tools/descriptions && cp -r src/tools/descriptions/* dist/tools/descriptions/",
     "start": "node dist/index.js",
     "dev": "tsx src/index.ts",
     "lint": "eslint src/**/*.ts"