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/store/app-store.ts

@@ -108,6 +108,9 @@ export interface Feature {
   model?: AgentModel; // Model to use for this feature (defaults to opus)
   thinkingLevel?: ThinkingLevel; // Thinking level for extended thinking (defaults to none)
   error?: string; // Error message if the agent errored during processing
+  // Worktree info - set when a feature is being worked on in an isolated git worktree
+  worktreePath?: string; // Path to the worktree directory
+  branchName?: string; // Name of the feature branch
 }
 
 export interface AppState {