add github sync flow

.gitea/workflows/github-sync.yaml

@@ -0,0 +1,39 @@
+name: Mirror to GitHub
+
+on:
+  push:
+
+jobs:
+  mirror:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Configure git
+        run: |
+          git config user.name "gitea-mirror"
+          git config user.email "mirror@example.com"
+
+      - name: Add GitHub remote
+        env:
+          MIRROR_REPO: ${{ vars.MIRROR_REPO }}
+          MIRROR_TOKEN: ${{ secrets.MIRROR_TOKEN }}
+        run: |
+          if [ -z "$MIRROR_REPO" ] || [ -z "$MIRROR_TOKEN" ]; then
+            echo "Missing required secrets: MIRROR_REPO and/or MIRROR_TOKEN"
+            exit 1
+          fi
+          git remote add github "https://x-access-token:${MIRROR_TOKEN}@github.com/${MIRROR_REPO}.git" || \
+            git remote set-url github "https://x-access-token:${MIRROR_TOKEN}@github.com/${MIRROR_REPO}.git"
+
+      - name: Push branches
+        run: |
+          git for-each-ref --format='%(refname:short)' refs/heads | while read branch; do
+            git push --force --prune github "refs/heads/$branch:refs/heads/$branch" || exit 1
+          done
+
+      - name: Push tags
+        run: git push --tags github --force
+