Set up GitHub Codespaces prebuilds for a repository (triggers, regions, retained versions, billing)

Documented steps

1. Prerequisites: GitHub Actions enabled for the repo (prebuilds run as Actions workflows); for organization-owned repos, Codespaces must be enabled/billable for the org
2. Go to repo Settings > Codespaces ('Code, planning, and automation' section) and click 'Set up prebuild'
3. Pick the branch to prebuild (usually the default branch) and, if the repo has multiple dev container configs, the devcontainer.json file to use
4. Choose the trigger: 'Every push' (freshest, most Actions minutes), 'On configuration change' (rebuild only when .devcontainer files change), or 'Scheduled' (cron-style)
5. Optionally restrict prebuilds to specific regions and set how many prebuild versions to retain; each region and retained version adds storage
6. Click 'Create'; watch the first prebuild workflow run under the Actions tab, then confirm new codespaces show the 'prebuild ready' fast path
7. Docs: https://docs.github.com/en/codespaces/prebuilding-your-codespaces/configuring-prebuilds