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/electron/services/feature-loader.js

@@ -141,6 +141,13 @@ class FeatureLoader {
       if (f.error !== undefined) {
         featureData.error = f.error;
       }
+      // Preserve worktree info
+      if (f.worktreePath !== undefined) {
+        featureData.worktreePath = f.worktreePath;
+      }
+      if (f.branchName !== undefined) {
+        featureData.branchName = f.branchName;
+      }
       return featureData;
     });
 
@@ -163,6 +170,69 @@ class FeatureLoader {
     // Skip verified and waiting_approval (which needs user input)
     return features.find((f) => f.status !== "verified" && f.status !== "waiting_approval");
   }
+
+  /**
+   * Update worktree info for a feature
+   * @param {string} featureId - The ID of the feature to update
+   * @param {string} projectPath - Path to the project
+   * @param {string|null} worktreePath - Path to the worktree (null to clear)
+   * @param {string|null} branchName - Name of the feature branch (null to clear)
+   */
+  async updateFeatureWorktree(featureId, projectPath, worktreePath, branchName) {
+    const featuresPath = path.join(
+      projectPath,
+      ".automaker",
+      "feature_list.json"
+    );
+
+    const features = await this.loadFeatures(projectPath);
+
+    if (!Array.isArray(features) || features.length === 0) {
+      console.error("[FeatureLoader] Cannot update worktree: feature list is empty");
+      return;
+    }
+
+    const feature = features.find((f) => f.id === featureId);
+
+    if (!feature) {
+      console.error(`[FeatureLoader] Feature ${featureId} not found`);
+      return;
+    }
+
+    // Update or clear worktree info
+    if (worktreePath) {
+      feature.worktreePath = worktreePath;
+      feature.branchName = branchName;
+    } else {
+      delete feature.worktreePath;
+      delete feature.branchName;
+    }
+
+    // Save back to file (reuse the same mapping logic)
+    const toSave = features.map((f) => {
+      const featureData = {
+        id: f.id,
+        category: f.category,
+        description: f.description,
+        steps: f.steps,
+        status: f.status,
+      };
+      if (f.skipTests !== undefined) featureData.skipTests = f.skipTests;
+      if (f.images !== undefined) featureData.images = f.images;
+      if (f.imagePaths !== undefined) featureData.imagePaths = f.imagePaths;
+      if (f.startedAt !== undefined) featureData.startedAt = f.startedAt;
+      if (f.summary !== undefined) featureData.summary = f.summary;
+      if (f.model !== undefined) featureData.model = f.model;
+      if (f.thinkingLevel !== undefined) featureData.thinkingLevel = f.thinkingLevel;
+      if (f.error !== undefined) featureData.error = f.error;
+      if (f.worktreePath !== undefined) featureData.worktreePath = f.worktreePath;
+      if (f.branchName !== undefined) featureData.branchName = f.branchName;
+      return featureData;
+    });
+
+    await fs.writeFile(featuresPath, JSON.stringify(toSave, null, 2), "utf-8");
+    console.log(`[FeatureLoader] Updated feature ${featureId}: worktreePath=${worktreePath}, branchName=${branchName}`);
+  }
 }
 
 module.exports = new FeatureLoader();