feat: AtchaV2 Phase 13 — 알람 이후 세션 수명 완결 (departed + stopIntent + 클라 자체 만료) - #351

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/