docs: Add SDK-first guidance to AGENTS.md and llms.txt - #5997

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@6ccd57f4c5d15bdc2fef309bd9fb6cc9db2ef1c6, github/codeql-action/analyze@4b1d7da102ff94aca014c0245062b1a463356d72, github/codeql-action/init@4b1d7da102ff94aca014c0245062b1a463356d72. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/