test: 补充单元测试提升代码覆盖率

新增测试文件:
- agent/executor-extended.test.ts, presets/
- context/manager-extended.test.ts
- core/agent.test.ts, providers.test.ts
- lsp/cli.test.ts, client-extended.test.ts, index.test.ts
- permission/file-prompt.test.ts, prompt.test.ts
- skills/builtin/
- tools/filesystem/write_file-extended.test.ts
- tools/git/git_commit-extended.test.ts
- tools/load_description.test.ts
- tools/todo/todo-manager.test.ts
- tools/tool-search.test.ts
- types/
- utils/config-extended.test.ts, diff-extended.test.ts

修改现有测试:
- agent/manager.test.ts
- tools/skill/skill.test.ts
- utils/config.test.ts, diff.test.ts, image.test.ts

tests/unit/utils/diff.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { computeDiff, countChanges, formatEditDiff } from '../../../src/utils/diff.js';
+import { computeDiff, countChanges, formatEditDiff, formatDiff } from '../../../src/utils/diff.js';
 
 describe('computeDiff - 计算文件差异', () => {
   describe('新文件', () => {
@@ -248,6 +248,56 @@ describe('LCS 算法测试', () => {
   });
 });
 
+describe('formatDiff - 格式化 diff 输出', () => {
+  it('新文件显示新增标记', () => {
+    const diff = computeDiff(null, 'line1\nline2');
+    const result = formatDiff(diff, '/test/file.ts');
+
+    expect(result).toContain('新文件');
+    expect(result).toContain('/test/file.ts');
+  });
+
+  it('修改文件显示原文件和修改后标记', () => {
+    const diff = computeDiff('old', 'new');
+    const result = formatDiff(diff, '/test/file.ts');
+
+    expect(result).toContain('原文件');
+    expect(result).toContain('修改后');
+    expect(result).toContain('/test/file.ts');
+  });
+
+  it('hunk 头部显示正确的行号范围', () => {
+    const diff = computeDiff('line1\nline2', 'line1\nnew\nline2');
+    const result = formatDiff(diff, '/test/file.ts');
+
+    expect(result).toContain('@@');
+  });
+
+  it('新增行显示 + 前缀', () => {
+    const diff = computeDiff(null, 'added line');
+    const result = formatDiff(diff, '/test/file.ts');
+
+    expect(result).toContain('+');
+    expect(result).toContain('added line');
+  });
+
+  it('删除行显示 - 前缀', () => {
+    const diff = computeDiff('removed line', 'new line');
+    const result = formatDiff(diff, '/test/file.ts');
+
+    expect(result).toContain('-');
+    expect(result).toContain('removed line');
+  });
+
+  it('空 diff 返回基本格式', () => {
+    const diff = computeDiff('same', 'same');
+    const result = formatDiff(diff, '/test/file.ts');
+
+    // 没有 hunks 时只有头部
+    expect(result).toContain('/test/file.ts');
+  });
+});
+
 describe('实际代码场景', () => {
   it('函数修改', () => {
     const oldContent = `function hello() {