feat: 补全 auto mode 分类器 prompt 模板，支持 FEATURE_* 环境变量注入

- 重建 yolo-classifier-prompts/ 三个缺失的 prompt 文件
- dev.ts/build.ts 扫描 FEATURE_* 环境变量注入 Bun --feature
- AUTO_MODE_ENABLED_DEFAULT 由 feature flag 决定，开 feature 即开 auto mode
- 补充 docs/safety/auto-mode.mdx prompt 模板章节

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/entrypoints/cli.tsx

@@ -1,6 +1,12 @@
 #!/usr/bin/env bun
 import { feature } from 'bun:bundle';
 
+// eslint-disable-next-line custom-rules/no-top-level-side-effects
+if (feature('TRANSCRIPT_CLASSIFIER')) {
+  // eslint-disable-next-line custom-rules/no-console, custom-rules/no-top-level-side-effects
+  console.log('[dev] feature TRANSCRIPT_CLASSIFIER enabled');
+}
+
 // Bugfix for corepack auto-pinning, which adds yarnpkg to peoples' package.jsons
 // eslint-disable-next-line custom-rules/no-top-level-side-effects
 process.env.COREPACK_ENABLE_AUTO_PIN = '0';