feat(core): implement git worktree checkpoint system

Add comprehensive worktree management system to enable task isolation and rollback capabilities. This allows users to revert agent changes if they don't satisfy requirements or break functionality.

Key components:
- New WorktreeManager service for branch and worktree operations
- GitDiffPanel component for visualizing changes
- Enhanced UI components with worktree integration
- Auto-mode service enhancements for worktree workflow

Modified files: worktree-manager.js, git-diff-panel.tsx, main.js, preload.js, feature-loader.js, agent-output-modal.tsx, board-view.tsx, kanban-card.tsx, electron.ts, app-store.ts, electron.d.ts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

app/src/types/electron.d.ts

@@ -383,6 +383,91 @@ export interface ElectronAPI {
     message?: string;
     error?: string;
   }>;
+
+  // Worktree Management APIs
+  worktree: WorktreeAPI;
+}
+
+export interface WorktreeInfo {
+  worktreePath: string;
+  branchName: string;
+  head?: string;
+  baseBranch?: string;
+}
+
+export interface WorktreeStatus {
+  success: boolean;
+  modifiedFiles?: number;
+  files?: string[];
+  diffStat?: string;
+  recentCommits?: string[];
+  error?: string;
+}
+
+export interface FileStatus {
+  status: string;
+  path: string;
+  statusText: string;
+}
+
+export interface FileDiffsResult {
+  success: boolean;
+  diff?: string;
+  files?: FileStatus[];
+  hasChanges?: boolean;
+  error?: string;
+}
+
+export interface FileDiffResult {
+  success: boolean;
+  diff?: string;
+  filePath?: string;
+  error?: string;
+}
+
+export interface WorktreeAPI {
+  // Revert feature changes by removing the worktree
+  revertFeature: (projectPath: string, featureId: string) => Promise<{
+    success: boolean;
+    removedPath?: string;
+    error?: string;
+  }>;
+
+  // Merge feature worktree changes back to main branch
+  mergeFeature: (projectPath: string, featureId: string, options?: {
+    squash?: boolean;
+    commitMessage?: string;
+    squashMessage?: string;
+  }) => Promise<{
+    success: boolean;
+    mergedBranch?: string;
+    error?: string;
+  }>;
+
+  // Get worktree info for a feature
+  getInfo: (projectPath: string, featureId: string) => Promise<{
+    success: boolean;
+    worktreePath?: string;
+    branchName?: string;
+    head?: string;
+    error?: string;
+  }>;
+
+  // Get worktree status (changed files, commits)
+  getStatus: (projectPath: string, featureId: string) => Promise<WorktreeStatus>;
+
+  // List all feature worktrees
+  list: (projectPath: string) => Promise<{
+    success: boolean;
+    worktrees?: WorktreeInfo[];
+    error?: string;
+  }>;
+
+  // Get file diffs for a feature worktree
+  getDiffs: (projectPath: string, featureId: string) => Promise<FileDiffsResult>;
+
+  // Get diff for a specific file in a worktree
+  getFileDiff: (projectPath: string, featureId: string, filePath: string) => Promise<FileDiffResult>;
 }
 
 // Model definition type