fix: [#285] correct RAG optional-dependency install instructions (cofone -> fenn[rag-*]) - #286

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/setup-python@0a5c61591373683505ea898e09a3ea4f39ef2b9c. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/