Add agent guidance, review rules, and a comment-hygiene gate - #514

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea, lukka/get-cmake@v4.2.3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/