Replaces the dev/beta branch split with one trunk (main): dev-track
builds still publish on every push, but the beta track now publishes
from a vX.Y.Z-rc.N prerelease tag instead of a separately-maintained
beta branch. Removes the branch nobody reliably kept in sync.
GitHub Actions self-hosted runners need per-repo registration on a
personal account; Forgejo Actions' runner already serves every repo
with zero setup. Moves release publishing there (dl.breadway.dev stays
the primary bakery target; GitHub release upload is kept as the
fallback via an explicit token, since Forgejo Actions has no ambient
GITHUB_TOKEN) and adds a mirror workflow to keep GitHub in sync
automatically.
