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/lib/electron.ts

@@ -42,7 +42,7 @@ export interface StatResult {
 }
 
 // Auto Mode types - Import from electron.d.ts to avoid duplication
-import type { AutoModeEvent, ModelDefinition, ProviderStatus } from "@/types/electron";
+import type { AutoModeEvent, ModelDefinition, ProviderStatus, WorktreeAPI, WorktreeInfo, WorktreeStatus, FileDiffsResult, FileDiffResult, FileStatus } from "@/types/electron";
 
 export interface AutoModeAPI {
   start: (projectPath: string, maxConcurrency?: number) => Promise<{ success: boolean; error?: string }>;
@@ -128,6 +128,7 @@ export interface ElectronAPI {
     message?: string;
     error?: string;
   }>;
+  worktree?: WorktreeAPI;
 }
 
 declare global {
@@ -422,9 +423,78 @@ export const getElectronAPI = (): ElectronAPI => {
 
     // Mock Auto Mode API
     autoMode: createMockAutoModeAPI(),
+
+    // Mock Worktree API
+    worktree: createMockWorktreeAPI(),
   };
 };
 
+// Mock Worktree API implementation
+function createMockWorktreeAPI(): WorktreeAPI {
+  return {
+    revertFeature: async (projectPath: string, featureId: string) => {
+      console.log("[Mock] Reverting feature:", { projectPath, featureId });
+      return { success: true, removedPath: `/mock/worktree/${featureId}` };
+    },
+
+    mergeFeature: async (projectPath: string, featureId: string, options?: object) => {
+      console.log("[Mock] Merging feature:", { projectPath, featureId, options });
+      return { success: true, mergedBranch: `feature/${featureId}` };
+    },
+
+    getInfo: async (projectPath: string, featureId: string) => {
+      console.log("[Mock] Getting worktree info:", { projectPath, featureId });
+      return {
+        success: true,
+        worktreePath: `/mock/worktrees/${featureId}`,
+        branchName: `feature/${featureId}`,
+        head: "abc1234",
+      };
+    },
+
+    getStatus: async (projectPath: string, featureId: string) => {
+      console.log("[Mock] Getting worktree status:", { projectPath, featureId });
+      return {
+        success: true,
+        modifiedFiles: 3,
+        files: ["src/feature.ts", "tests/feature.spec.ts", "README.md"],
+        diffStat: " 3 files changed, 50 insertions(+), 10 deletions(-)",
+        recentCommits: [
+          "abc1234 feat: implement feature",
+          "def5678 test: add tests for feature",
+        ],
+      };
+    },
+
+    list: async (projectPath: string) => {
+      console.log("[Mock] Listing worktrees:", { projectPath });
+      return { success: true, worktrees: [] };
+    },
+
+    getDiffs: async (projectPath: string, featureId: string) => {
+      console.log("[Mock] Getting file diffs:", { projectPath, featureId });
+      return {
+        success: true,
+        diff: "diff --git a/src/feature.ts b/src/feature.ts\n+++ new file\n@@ -0,0 +1,10 @@\n+export function feature() {\n+  return 'hello';\n+}",
+        files: [
+          { status: "A", path: "src/feature.ts", statusText: "Added" },
+          { status: "M", path: "README.md", statusText: "Modified" },
+        ],
+        hasChanges: true,
+      };
+    },
+
+    getFileDiff: async (projectPath: string, featureId: string, filePath: string) => {
+      console.log("[Mock] Getting file diff:", { projectPath, featureId, filePath });
+      return {
+        success: true,
+        diff: `diff --git a/${filePath} b/${filePath}\n+++ new file\n@@ -0,0 +1,5 @@\n+// New content`,
+        filePath,
+      };
+    },
+  };
+}
+
 // Mock Auto Mode state and implementation
 let mockAutoModeRunning = false;
 let mockRunningFeatures = new Set<string>(); // Track multiple concurrent feature verifications