refactor: own the CGO-free switch and guard the surface in core (#345) - #347

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, actions/setup-go@v3, astral-sh/setup-uv@v5, slackapi/slack-github-action@v1.24.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/