[fix] Make checkpoint replacement atomic via tmp/old rename sequence - #155

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, actions/setup-python@0b93645e9fea7318ecaed2b359559ac225c90a2b. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/