feat: add takt reset config with backup restore

docs/README.ja.md

@@ -294,6 +294,9 @@ takt prompt [piece]
 
 # ピースカテゴリをビルトインのデフォルトにリセット
 takt reset categories
+
+# グローバル config をテンプレートにリセット（旧設定は .old に退避）
+takt reset config
 ```
 
 ### おすすめピース

docs/testing/e2e.md

@@ -151,6 +151,15 @@ E2Eテストを追加・変更した場合は、このドキュメントも更
     - `takt reset categories` を実行する。
     - 出力に `reset` を含むことを確認する。
     - `$TAKT_CONFIG_DIR/preferences/piece-categories.yaml` が存在し `piece_categories: {}` を含むことを確認する。
+- Reset config（`e2e/specs/cli-reset-config.e2e.ts`）
+  - 目的: `takt reset config` でグローバル設定をテンプレートへ戻し、旧設定をバックアップすることを確認。
+  - LLM: 呼び出さない（LLM不使用の操作のみ）
+  - 手順（ユーザー行動/コマンド）:
+    - `$TAKT_CONFIG_DIR/config.yaml` に任意の設定を書き込む（例: `language: ja`, `provider: mock`）。
+    - `takt reset config` を実行する。
+    - 出力に `reset` と `backup:` を含むことを確認する。
+    - `$TAKT_CONFIG_DIR/config.yaml` がテンプレート内容（例: `branch_name_strategy: ai`, `concurrency: 2`）に置き換わっていることを確認する。
+    - `$TAKT_CONFIG_DIR/` 直下に `config.yaml.YYYYMMDD-HHmmss.old` 形式のバックアップファイルが1件作成されることを確認する。
 - Export Claude Code Skill（`e2e/specs/cli-export-cc.e2e.ts`）
   - 目的: `takt export-cc` でClaude Code Skillのデプロイを確認。
   - LLM: 呼び出さない（LLM不使用の操作のみ）

e2e/specs/cli-reset-config.e2e.ts

@@ -0,0 +1,48 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { readdirSync, readFileSync, writeFileSync } from 'node:fs';
+import { join } from 'node:path';
+import { createIsolatedEnv, type IsolatedEnv } from '../helpers/isolated-env';
+import { runTakt } from '../helpers/takt-runner';
+import { createLocalRepo, type LocalRepo } from '../helpers/test-repo';
+
+// E2E更新時は docs/testing/e2e.md も更新すること
+describe('E2E: Reset config command (takt reset config)', () => {
+  let isolatedEnv: IsolatedEnv;
+  let repo: LocalRepo;
+
+  beforeEach(() => {
+    isolatedEnv = createIsolatedEnv();
+    repo = createLocalRepo();
+  });
+
+  afterEach(() => {
+    try { repo.cleanup(); } catch { /* best-effort */ }
+    try { isolatedEnv.cleanup(); } catch { /* best-effort */ }
+  });
+
+  it('should backup current config and replace with builtin template', () => {
+    const configPath = join(isolatedEnv.taktDir, 'config.yaml');
+    writeFileSync(configPath, ['language: ja', 'provider: mock'].join('\n'), 'utf-8');
+
+    const result = runTakt({
+      args: ['reset', 'config'],
+      cwd: repo.path,
+      env: isolatedEnv.env,
+    });
+
+    expect(result.exitCode).toBe(0);
+    const output = result.stdout;
+    expect(output).toMatch(/reset/i);
+    expect(output).toMatch(/backup:/i);
+
+    const config = readFileSync(configPath, 'utf-8');
+    expect(config).toContain('language: ja');
+    expect(config).toContain('branch_name_strategy: ai');
+    expect(config).toContain('concurrency: 2');
+
+    const backups = readdirSync(isolatedEnv.taktDir).filter((name) =>
+      /^config\.yaml\.\d{8}-\d{6}\.old(\.\d+)?$/.test(name),
+    );
+    expect(backups.length).toBe(1);
+  });
+});

src/__tests__/reset-global-config.test.ts

@@ -0,0 +1,54 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { mkdtempSync, mkdirSync, readFileSync, writeFileSync, existsSync, rmSync } from 'node:fs';
+import { join } from 'node:path';
+import { tmpdir } from 'node:os';
+import { resetGlobalConfigToTemplate } from '../infra/config/global/resetConfig.js';
+
+describe('resetGlobalConfigToTemplate', () => {
+  const originalEnv = process.env;
+  let testRoot: string;
+  let taktDir: string;
+  let configPath: string;
+
+  beforeEach(() => {
+    testRoot = mkdtempSync(join(tmpdir(), 'takt-reset-config-'));
+    taktDir = join(testRoot, '.takt');
+    mkdirSync(taktDir, { recursive: true });
+    configPath = join(taktDir, 'config.yaml');
+    process.env = { ...originalEnv, TAKT_CONFIG_DIR: taktDir };
+  });
+
+  afterEach(() => {
+    process.env = originalEnv;
+    rmSync(testRoot, { recursive: true, force: true });
+  });
+
+  it('should backup existing config and replace with language-matched template', () => {
+    writeFileSync(configPath, ['language: ja', 'provider: mock'].join('\n'), 'utf-8');
+
+    const result = resetGlobalConfigToTemplate(new Date('2026-02-19T12:00:00Z'));
+
+    expect(result.language).toBe('ja');
+    expect(result.backupPath).toBeDefined();
+    expect(existsSync(result.backupPath!)).toBe(true);
+    expect(readFileSync(result.backupPath!, 'utf-8')).toContain('provider: mock');
+
+    const newConfig = readFileSync(configPath, 'utf-8');
+    expect(newConfig).toContain('language: ja');
+    expect(newConfig).toContain('branch_name_strategy: ai');
+    expect(newConfig).toContain('concurrency: 2');
+  });
+
+  it('should create config from default language template when config does not exist', () => {
+    rmSync(configPath, { force: true });
+
+    const result = resetGlobalConfigToTemplate(new Date('2026-02-19T12:00:00Z'));
+
+    expect(result.backupPath).toBeUndefined();
+    expect(result.language).toBe('en');
+    expect(existsSync(configPath)).toBe(true);
+    const newConfig = readFileSync(configPath, 'utf-8');
+    expect(newConfig).toContain('language: en');
+    expect(newConfig).toContain('branch_name_strategy: ai');
+  });
+});

src/app/cli/commands.ts

@@ -7,7 +7,7 @@
 import { clearPersonaSessions, resolveConfigValue } from '../../infra/config/index.js';
 import { success } from '../../shared/ui/index.js';
 import { runAllTasks, addTask, watchTasks, listTasks } from '../../features/tasks/index.js';
-import { switchPiece, ejectBuiltin, ejectFacet, parseFacetType, VALID_FACET_TYPES, resetCategoriesToDefault, deploySkill } from '../../features/config/index.js';
+import { switchPiece, ejectBuiltin, ejectFacet, parseFacetType, VALID_FACET_TYPES, resetCategoriesToDefault, resetConfigToDefault, deploySkill } from '../../features/config/index.js';
 import { previewPrompts } from '../../features/prompt/index.js';
 import { showCatalog } from '../../features/catalog/index.js';
 import { program, resolvedCwd } from './program.js';
@@ -100,6 +100,13 @@ const reset = program
   .command('reset')
   .description('Reset settings to defaults');
 
+reset
+  .command('config')
+  .description('Reset global config to builtin template (with backup)')
+  .action(async () => {
+    await resetConfigToDefault();
+  });
+
 reset
   .command('categories')
   .description('Reset piece categories to builtin defaults')

src/features/config/index.ts

@@ -5,4 +5,5 @@
 export { switchPiece } from './switchPiece.js';
 export { ejectBuiltin, ejectFacet, parseFacetType, VALID_FACET_TYPES } from './ejectBuiltin.js';
 export { resetCategoriesToDefault } from './resetCategories.js';
+export { resetConfigToDefault } from './resetConfig.js';
 export { deploySkill } from './deploySkill.js';

src/features/config/resetConfig.ts

@@ -0,0 +1,13 @@
+import { resetGlobalConfigToTemplate } from '../../infra/config/global/index.js';
+import { header, info, success } from '../../shared/ui/index.js';
+
+export async function resetConfigToDefault(): Promise<void> {
+  header('Reset Config');
+
+  const result = resetGlobalConfigToTemplate();
+  success('Global config reset from builtin template.');
+  info(`  config: ${result.configPath}`);
+  if (result.backupPath) {
+    info(`  backup: ${result.backupPath}`);
+  }
+}

src/infra/config/global/index.ts

@@ -30,6 +30,11 @@ export {
   resetPieceCategories,
 } from './pieceCategories.js';
 
+export {
+  resetGlobalConfigToTemplate,
+  type ResetGlobalConfigResult,
+} from './resetConfig.js';
+
 export {
   needsLanguageSetup,
   promptLanguageSelection,

src/infra/config/global/resetConfig.ts

@@ -0,0 +1,71 @@
+import { copyFileSync, existsSync, mkdirSync, readFileSync, renameSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { parse as parseYaml } from 'yaml';
+import type { Language } from '../../../core/models/index.js';
+import { DEFAULT_LANGUAGE } from '../../../shared/constants.js';
+import { getLanguageResourcesDir } from '../../resources/index.js';
+import { getGlobalConfigPath } from '../paths.js';
+import { invalidateGlobalConfigCache } from './globalConfig.js';
+
+export interface ResetGlobalConfigResult {
+  configPath: string;
+  backupPath?: string;
+  language: Language;
+}
+
+function detectConfigLanguage(configPath: string): Language {
+  if (!existsSync(configPath)) return DEFAULT_LANGUAGE;
+  const raw = readFileSync(configPath, 'utf-8');
+  const parsed = parseYaml(raw) as { language?: unknown } | null;
+  if (parsed && typeof parsed !== 'object') {
+    throw new Error(`Invalid config format: ${configPath} must be a YAML object.`);
+  }
+  const language = parsed?.language;
+  if (language === undefined) return DEFAULT_LANGUAGE;
+  if (language === 'ja' || language === 'en') return language;
+  throw new Error(`Invalid language in ${configPath}: ${String(language)} (expected: ja | en)`);
+}
+
+function formatTimestamp(date: Date): string {
+  const y = String(date.getFullYear());
+  const m = String(date.getMonth() + 1).padStart(2, '0');
+  const d = String(date.getDate()).padStart(2, '0');
+  const hh = String(date.getHours()).padStart(2, '0');
+  const mm = String(date.getMinutes()).padStart(2, '0');
+  const ss = String(date.getSeconds()).padStart(2, '0');
+  return `${y}${m}${d}-${hh}${mm}${ss}`;
+}
+
+function resolveBackupPath(configPath: string, timestamp: string): string {
+  const base = `${configPath}.${timestamp}.old`;
+  if (!existsSync(base)) return base;
+  let index = 1;
+  while (true) {
+    const candidate = `${base}.${index}`;
+    if (!existsSync(candidate)) return candidate;
+    index += 1;
+  }
+}
+
+export function resetGlobalConfigToTemplate(now = new Date()): ResetGlobalConfigResult {
+  const configPath = getGlobalConfigPath();
+  const configDir = dirname(configPath);
+  mkdirSync(configDir, { recursive: true });
+
+  const language = detectConfigLanguage(configPath);
+  const templatePath = join(getLanguageResourcesDir(language), 'config.yaml');
+  if (!existsSync(templatePath)) {
+    throw new Error(`Builtin config template not found: ${templatePath}`);
+  }
+
+  let backupPath: string | undefined;
+  if (existsSync(configPath)) {
+    backupPath = resolveBackupPath(configPath, formatTimestamp(now));
+    renameSync(configPath, backupPath);
+  }
+
+  copyFileSync(templatePath, configPath);
+  invalidateGlobalConfigCache();
+
+  return { configPath, backupPath, language };
+}