refactor: 알람 세션 단일 소유 + 트리거·표출 분리 (Phase B3c·C·D) - #368

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. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/