Add GitHub Actions workflow that publishes to GHCR
Mirrors the Forgejo workflow's behavior — test on every push/PR, build
image on pushes to main — but uses GitHub-native idioms: stock
actions/checkout, astral-sh/setup-uv, docker/login-action, and
docker/build-push-action. Auth is the built-in GITHUB_TOKEN with
packages: write permission, so no secrets or repo vars need to be set
on the mirrored GitHub repo. Image lands at
ghcr.io/<owner>/spacebee:latest (and :sha); the first publish is
private by default and can be flipped to public in GitHub's package
settings.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>