chore(ci): mirror to GitHub with --mirror so deletes propagate

Today's cleanup surfaced 14 branches that existed on GitHub but had
been deleted on Gitea — the previous `--all --force` + `--tags --force`
pair pushes refs but never deletes missing ones, so the mirror drifted
over time.

Switching to `git push --mirror` (equivalent to --all --tags --prune
--force) makes the GitHub side a true reflection of Gitea: branch and
tag deletes propagate automatically.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/mirror-to-github.yml

@@ -15,5 +15,4 @@ jobs:
           git clone --mirror https://gitea.resolutionflow.com/chihlasm/resolutionflow.git repo
           cd repo
           git remote add github https://x-access-token:${{ secrets.GH_MIRROR_TOKEN }}@github.com/${{ secrets.GH_MIRROR_REPO }}
-          git push github --all --force
-          git push github --tags --force
+          git push github --mirror