Adds .g1t/workflows/ci.yml, a workflow named CI that runs on every pull request, on pushes to main, and on the merge queue. g1t could not tell how this project is built, so the job is a placeholder: replace its last step with the commands that build and test it.
Its runs are this repository's checks: they show on every pull request, whoever opened it, and an agent whose change fails them is sent back with what the failing jobs printed.
Once this has run, require CI in Settings, Branches and merging, so that nothing merges into main unless it passes.