Add GitHub mirror workflow

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

@@ -0,0 +1,24 @@
+name: Mirror to GitHub
+
+on:
+  push:
+    branches:
+      - '**'
+  delete:
+    branches:
+      - '**'
+
+jobs:
+  mirror:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Push to GitHub
+        run: |
+          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