Add opt-in checkpoint retention policy (TASK-049) - #84

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, astral-sh/setup-uv@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Unexpected input(s) 'python-version-file', valid inputs are ['version', 'pyproject-file', 'uv-file', 'python-version', 'checksum', 'github-token', 'enable-cache', 'cache-dependency-glob', 'cache-suffix', 'cache-local-path', 'prune-cache', 'ignore-nothing-to-cache', 'ignore-empty-workdir', 'tool-dir', 'tool-bin-dir']