Add GitHub Actions workflow to mirror to GitLab

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

@@ -0,0 +1,33 @@
+name: Mirror to GitLab
+
+on:
+  push:
+    branches: ['**']
+    tags: ['**']
+  delete:
+  workflow_dispatch:
+
+concurrency:
+  group: mirror-to-gitlab
+  cancel-in-progress: false
+
+jobs:
+  mirror:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout full history
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Setup SSH
+        run: |
+          mkdir -p ~/.ssh
+          echo "${{ secrets.GITLAB_SSH_PRIVATE_KEY }}" > ~/.ssh/id_ed25519
+          chmod 600 ~/.ssh/id_ed25519
+          ssh-keyscan -t rsa,ecdsa,ed25519 gitlab.com >> ~/.ssh/known_hosts
+
+      - name: Push to GitLab
+        run: |
+          git remote add gitlab "${{ secrets.GITLAB_REPO_URL }}"
+          git push --mirror gitlab