fix: expand untracked directories to show individual file diffs

Previously, when git status reported an untracked directory (e.g., "?? apps/"),
the code would try to read the directory as a file, which failed and showed
"[Unable to read file content]".

Now, when encountering a directory:
- Strip trailing slash from path (git reports dirs as "dirname/")
- Check if path is a directory using stats.isDirectory()
- Recursively list all files inside using listAllFilesInDirectory
- Generate synthetic diffs for each file found

This ensures users see the actual file contents in the diff view instead
of an error placeholder.

libs/git-utils/tests/diff.test.ts

@@ -118,7 +118,7 @@ describe('diff.ts', () => {
       expect(diff).toContain('[Unable to read file content]');
     });
 
-    it('should handle directory path gracefully', async () => {
+    it('should handle empty directory path gracefully', async () => {
       const dirName = 'some-directory';
       const dirPath = path.join(tempDir, dirName);
       await fs.mkdir(dirPath);
@@ -127,7 +127,38 @@ describe('diff.ts', () => {
 
       expect(diff).toContain(`diff --git a/${dirName} b/${dirName}`);
       expect(diff).toContain('new file mode 040000');
-      expect(diff).toContain('[Directory]');
+      expect(diff).toContain('[Empty directory]');
+    });
+
+    it('should expand directory with files and generate diffs for each file', async () => {
+      const dirName = 'new-feature';
+      const dirPath = path.join(tempDir, dirName);
+      await fs.mkdir(dirPath);
+      await fs.writeFile(path.join(dirPath, 'index.ts'), 'export const foo = 1;\n');
+      await fs.writeFile(path.join(dirPath, 'utils.ts'), 'export const bar = 2;\n');
+
+      const diff = await generateSyntheticDiffForNewFile(tempDir, dirName);
+
+      // Should contain diffs for both files in the directory
+      expect(diff).toContain(`diff --git a/${dirName}/index.ts b/${dirName}/index.ts`);
+      expect(diff).toContain(`diff --git a/${dirName}/utils.ts b/${dirName}/utils.ts`);
+      expect(diff).toContain('+export const foo = 1;');
+      expect(diff).toContain('+export const bar = 2;');
+      // Should NOT contain a diff for the directory itself
+      expect(diff).not.toContain('[Empty directory]');
+    });
+
+    it('should handle directory path with trailing slash', async () => {
+      const dirName = 'trailing-slash-dir';
+      const dirPath = path.join(tempDir, dirName);
+      await fs.mkdir(dirPath);
+      await fs.writeFile(path.join(dirPath, 'file.txt'), 'content\n');
+
+      // git status reports untracked directories with trailing slash
+      const diff = await generateSyntheticDiffForNewFile(tempDir, `${dirName}/`);
+
+      expect(diff).toContain(`diff --git a/${dirName}/file.txt b/${dirName}/file.txt`);
+      expect(diff).toContain('+content');
     });
   });