feat(checkpoint): 添加 Checkpoint 可视化管理功能

Core 层增强:
- 添加 safety.ts: 7点安全检查机制
- 添加 session-tracker.ts: 会话级检查点跟踪
- 添加 lock.ts: 并发控制文件锁
- 添加 lfs.ts: Git LFS 大文件支持
- 添加 path-validator.ts: 路径验证
- 添加 commit-message.ts: 智能提交消息生成
- 增强 manager.ts: 支持三种恢复模式、unrevert 撤销回滚

Server 层:
- 添加 checkpoints.ts: 16个 REST API 端点
  - GET/POST /checkpoints: 列表/创建检查点
  - GET/DELETE /checkpoints/🆔 获取/删除检查点
  - GET /checkpoints/:id/diff: 获取差异
  - POST /checkpoints/:id/restore: 恢复到检查点
  - POST /checkpoints/unrevert: 撤销回滚
  - GET /checkpoints/:id/safety-check: 安全检查

UI 层:
- 添加 CheckpointPanel.tsx: 检查点列表面板
- 添加 CheckpointDiffViewer.tsx: 差异查看器
- 添加 RestoreDialog.tsx: 恢复确认对话框
- 添加 16 个 API 客户端函数
- 添加完整的 TypeScript 类型定义

Web/Desktop 集成:
- 添加 History 按钮到工具栏
- 集成 CheckpointPanel 组件

packages/core/src/index.ts

@@ -44,6 +44,53 @@ export type {
   CommandOperationResult,
 } from './commands/index.js';
 
+// Checkpoint
+export {
+  CheckpointManager,
+  getCheckpointManager,
+  initCheckpointManager,
+  resetCheckpointManager,
+  ShadowGit,
+  createShadowGit,
+  hashWorkingDir,
+  CheckpointSafetyChecker,
+  createSafetyChecker,
+  SessionTracker,
+  createSessionTracker,
+  CheckpointLock,
+  LFSPatternLoader,
+  createLFSPatternLoader,
+  isCommonLargeFile,
+  COMMON_LARGE_FILE_EXTENSIONS,
+  WorkspacePathValidator,
+  createPathValidator,
+  CommitMessageGenerator,
+  createCommitMessageGenerator,
+  RestoreMode,
+  DEFAULT_CHECKPOINT_CONFIG,
+} from './checkpoint/index.js';
+
+export type {
+  CheckpointMetadata,
+  CheckpointConfig,
+  CheckpointTrigger,
+  FileChange,
+  FileChangeType,
+  DiffInfo,
+  FileDiff,
+  RollbackOptions,
+  RollbackResult,
+  CheckpointEvent,
+  CheckpointEventType,
+  CheckpointEventListener,
+  RollbackRecord,
+  UnrevertResult,
+  SafetyCheckResult,
+  SessionState,
+  SessionStats,
+  PathValidationResult,
+} from './checkpoint/index.js';
+
 const program = new Command();
 
 // MCP 管理器实例