fix: eleven audit findings, and the standing cost measured and cut - #45

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: golangci/golangci-lint-action@4afd733a84b1f43292c63897423277bb7f4313a9. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/