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

@@ -0,0 +1,19 @@
+name: Mirror to GitHub
+
+on:
+  push:
+    branches: ['**']
+    tags: ['**']
+
+jobs:
+  mirror:
+    runs-on: [self-hosted, hestia]
+    steps:
+      - name: Mirror to GitHub
+        run: |
+          set -euo pipefail
+          git clone --mirror "https://git.breadway.dev/${GITHUB_REPOSITORY}.git" repo.git
+          cd repo.git
+          git push --prune \
+            "https://x-access-token:${{ secrets.MIRROR_TOKEN }}@github.com/Breadway/breadshot.git" \
+            '+refs/heads/*:refs/heads/*' '+refs/tags/*:refs/tags/*'