feat(interview): 面试工程化加固（single-flight 防重复扣费 + 追问规则短路 + 追问维度约束 + 评分防注入） - #4

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-python@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/