ci: remove stale GitHub push-mirror workflow

Already removed from dev in 1120729; main never got it, so it kept
triggering and failing on every push (MIRROR_TOKEN is expired — see
open issue tracked separately). GitHub stays in sync via manual
'git push github' alongside origin, per repo hygiene notes.

.forgejo/workflows/mirror.yml

@@ -1,21 +0,0 @@
-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/${GITHUB_REPOSITORY}.git" repo.git
-          cd repo.git
-          # Mirror only branches and tags (not refs/pull/*, which GitHub rejects);
-          # --prune deletes GitHub refs that no longer exist on Forgejo.
-          git push --prune \
-            "https://x-access-token:${{ secrets.MIRROR_TOKEN }}@github.com/Breadway/bos.git" \
-            '+refs/heads/*:refs/heads/*' '+refs/tags/*:refs/tags/*'