feat: AtchaV2 Phase 7 — CoreAlarm: AlarmKit 스케줄링 E2E (단일 알람 교체 정책) - #345

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/