Branch protection on the main branch

Currently we don't have branch protection warning users they're merging in a change that breaks the build. I'd like to add branch protection to the `main` branch preventing merge unless CI passes. This wouldn't actually slow down development because merging into main (currently) just fails silently and leaves production on the previous commit. Branch protection can be overwritten, but requires an extra prompt to ensure the user knows they're merging in something that will fail to deploy.

Start in the repository's branch protection settings for the main branch and inspect the existing CI checks. Done means merges to main require CI to pass while retaining an explicit override prompt for failing checks; verify the setting with a test change.

Written by the indexing model from the issue text.