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/components/views/board-view.tsx

@@ -1090,6 +1090,106 @@ export function BoardView() {
     });
   };
 
+  // Revert feature changes by removing the worktree
+  const handleRevertFeature = async (feature: Feature) => {
+    if (!currentProject) return;
+
+    console.log("[Board] Reverting feature:", {
+      id: feature.id,
+      description: feature.description,
+      branchName: feature.branchName,
+    });
+
+    try {
+      const api = getElectronAPI();
+      if (!api?.worktree?.revertFeature) {
+        console.error("Worktree API not available");
+        toast.error("Revert not available", {
+          description: "This feature is not available in the current version.",
+        });
+        return;
+      }
+
+      const result = await api.worktree.revertFeature(
+        currentProject.path,
+        feature.id
+      );
+
+      if (result.success) {
+        console.log("[Board] Feature reverted successfully");
+        // Reload features to update the UI
+        await loadFeatures();
+        toast.success("Feature reverted", {
+          description: `All changes discarded. Moved back to backlog: ${feature.description.slice(
+            0,
+            50
+          )}${feature.description.length > 50 ? "..." : ""}`,
+        });
+      } else {
+        console.error("[Board] Failed to revert feature:", result.error);
+        toast.error("Failed to revert feature", {
+          description: result.error || "An error occurred",
+        });
+      }
+    } catch (error) {
+      console.error("[Board] Error reverting feature:", error);
+      toast.error("Failed to revert feature", {
+        description:
+          error instanceof Error ? error.message : "An error occurred",
+      });
+    }
+  };
+
+  // Merge feature worktree changes back to main branch
+  const handleMergeFeature = async (feature: Feature) => {
+    if (!currentProject) return;
+
+    console.log("[Board] Merging feature:", {
+      id: feature.id,
+      description: feature.description,
+      branchName: feature.branchName,
+    });
+
+    try {
+      const api = getElectronAPI();
+      if (!api?.worktree?.mergeFeature) {
+        console.error("Worktree API not available");
+        toast.error("Merge not available", {
+          description: "This feature is not available in the current version.",
+        });
+        return;
+      }
+
+      const result = await api.worktree.mergeFeature(
+        currentProject.path,
+        feature.id
+      );
+
+      if (result.success) {
+        console.log("[Board] Feature merged successfully");
+        // Reload features to update the UI
+        await loadFeatures();
+        toast.success("Feature merged", {
+          description: `Changes merged to main branch: ${feature.description.slice(
+            0,
+            50
+          )}${feature.description.length > 50 ? "..." : ""}`,
+        });
+      } else {
+        console.error("[Board] Failed to merge feature:", result.error);
+        toast.error("Failed to merge feature", {
+          description: result.error || "An error occurred",
+        });
+      }
+    } catch (error) {
+      console.error("[Board] Error merging feature:", error);
+      toast.error("Failed to merge feature", {
+        description:
+          error instanceof Error ? error.message : "An error occurred",
+      });
+    }
+  };
+
   const checkContextExists = async (featureId: string): Promise<boolean> => {
     if (!currentProject) return false;
 
@@ -1463,6 +1563,8 @@ export function BoardView() {
                             }
                             onFollowUp={() => handleOpenFollowUp(feature)}
                             onCommit={() => handleCommitFeature(feature)}
+                            onRevert={() => handleRevertFeature(feature)}
+                            onMerge={() => handleMergeFeature(feature)}
                             hasContext={featuresWithContext.has(feature.id)}
                             isCurrentAutoTask={runningAutoTasks.includes(
                               feature.id