docs: document the typecheck gate and correct CLAUDE.md - #7

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, JohnnyMorganz/stylua-action@v4, leafo/gh-actions-lua@v11, leafo/gh-actions-luarocks@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

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gh-actions-lua: Storing into cache: lua:5.4.7:linux:x64: