exercise-builder

Use when a tutorial module plan exists but each module still needs a verifiable teaching loop. **Trigger**: exercises, practice, verification checklist, 教程练习, 可验证作业. **Use when**: tutorial 的 C2，已有 `outline/module_plan.yml`，需要为每个模块补齐 exercise / expected output / verification steps。 **Skip if**: 还没有 m