fix: harden pre-push checks and add tests - #14

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: 5monkeys/cobertura-action@ee5787cc56634acddedc51f21c7947985531e6eb. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/