CI: migrate release workflow from GitHub Actions to Forgejo Actions

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.

.forgejo/workflows/mirror.yml

@@ -14,8 +14,6 @@ jobs:
           set -euo pipefail
           git clone --mirror "https://git.breadway.dev/${GITHUB_REPOSITORY}.git" repo.git
           cd repo.git
-          # Mirror only branches and tags (not refs/pull/*, which GitHub rejects);
-          # --prune deletes GitHub refs that no longer exist on Forgejo.
           git push --prune \
             "https://x-access-token:${{ secrets.MIRROR_TOKEN }}@github.com/Breadway/bread.git" \
             '+refs/heads/*:refs/heads/*' '+refs/tags/*:refs/tags/*'