feat: 实现 Git 深度集成

参考 aider 的实现，添加 Git 深度集成功能：

- GitRepo: 封装 simple-git 的基础 Git 操作
- AutoCommitManager: 支持 immediate/batch/manual 三种自动提交模式
- MessageGenerator: 智能生成 conventional/simple/detailed 格式的 commit message
- UndoManager: 安全的 undo 机制，仅允许撤销 AI 生成的提交

主要特性:
- 文件变更后自动提交 (batch 模式默认 3 秒延迟)
- 智能检测变更类型 (feat/fix/docs/test/chore 等)
- 自动检测 scope (从文件路径推断)
- 多重安全检查的 undo 机制
- 追踪 AI 生成的提交，防止误撤销用户提交
- 与 Hook 系统集成

添加 simple-git 依赖
编写完整测试用例 (26 个测试)

package-lock.json

@@ -21,6 +21,7 @@
         "minimatch": "^10.1.1",
         "ora": "^8.1.0",
         "qwen-ai-provider-v5": "^1.0.2",
+        "simple-git": "^3.30.0",
         "tree-sitter-bash": "^0.25.1",
         "uuid": "^13.0.0",
         "vscode-jsonrpc": "^8.2.1",
@@ -1022,6 +1023,21 @@
         "@jridgewell/sourcemap-codec": "^1.4.14"
       }
     },
+    "node_modules/@kwsites/file-exists": {
+      "version": "1.1.1",
+      "resolved": "https://registry.npmjs.org/@kwsites/file-exists/-/file-exists-1.1.1.tgz",
+      "integrity": "sha512-m9/5YGR18lIwxSFDwfE3oA7bWuq9kdau6ugN4H2rJeyhFQZcG9AgSHkQtSD15a8WvTgfz9aikZMrKPHvbpqFiw==",
+      "license": "MIT",
+      "dependencies": {
+        "debug": "^4.1.1"
+      }
+    },
+    "node_modules/@kwsites/promise-deferred": {
+      "version": "1.1.1",
+      "resolved": "https://registry.npmjs.org/@kwsites/promise-deferred/-/promise-deferred-1.1.1.tgz",
+      "integrity": "sha512-GaHYm+c0O9MjZRu0ongGBRbinu8gVAMd2UZjji6jVmqKtZluZnptXGWhz1E8j8D2HJ3f/yMxKAUC0b+57wncIw==",
+      "license": "MIT"
+    },
     "node_modules/@opentelemetry/api": {
       "version": "1.9.0",
       "resolved": "https://registry.npmjs.org/@opentelemetry/api/-/api-1.9.0.tgz",
@@ -2780,6 +2796,21 @@
         "url": "https://github.com/sponsors/isaacs"
       }
     },
+    "node_modules/simple-git": {
+      "version": "3.30.0",
+      "resolved": "https://registry.npmjs.org/simple-git/-/simple-git-3.30.0.tgz",
+      "integrity": "sha512-q6lxyDsCmEal/MEGhP1aVyQ3oxnagGlBDOVSIB4XUVLl1iZh0Pah6ebC9V4xBap/RfgP2WlI8EKs0WS0rMEJHg==",
+      "license": "MIT",
+      "dependencies": {
+        "@kwsites/file-exists": "^1.1.1",
+        "@kwsites/promise-deferred": "^1.1.1",
+        "debug": "^4.4.0"
+      },
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/steveukx/git-js?sponsor=1"
+      }
+    },
     "node_modules/source-map-js": {
       "version": "1.2.1",
       "resolved": "https://registry.npmjs.org/source-map-js/-/source-map-js-1.2.1.tgz",