feat: phase 3 for git integration

tests/engine/GitEngine.test.ts

@@ -4,6 +4,7 @@ const mockVersion = vi.fn();
 const mockCheckIsRepo = vi.fn();
 const mockRevparse = vi.fn();
 const mockStatus = vi.fn();
+const mockDiff = vi.fn();
 const mockInit = vi.fn();
 const mockRaw = vi.fn();
 const mockAdd = vi.fn();
@@ -30,6 +31,7 @@ vi.mock('simple-git', () => ({
     checkIsRepo: mockCheckIsRepo,
     revparse: mockRevparse,
     status: mockStatus,
+    diff: mockDiff,
     init: mockInit,
     raw: mockRaw,
     add: mockAdd,
@@ -139,6 +141,20 @@ describe('GitEngine', () => {
     });
   });
 
+  describe('getDiff', () => {
+    it('should return patch output for a repository file', async () => {
+      mockDiff.mockResolvedValue('diff --git a/posts/first.md b/posts/first.md\n+hello');
+
+      const result = await gitEngine.getDiff('/tmp/project', 'posts/first.md');
+
+      expect(mockDiff).toHaveBeenCalledWith(['--', 'posts/first.md']);
+      expect(result).toEqual({
+        filePath: 'posts/first.md',
+        patch: 'diff --git a/posts/first.md b/posts/first.md\n+hello',
+      });
+    });
+  });
+
   describe('ensureGitignore', () => {
     it('should create .gitignore with default system metadata entries when missing', async () => {
       mockReadFile.mockRejectedValue(new Error('ENOENT'));