ci: fix mirror workflow – remove actions/checkout (needs Node.js)

Replace JS action with plain git clone --bare + push --mirror.
No Node.js required → runs directly on the Pi host without a container.
Also upgrades from single-branch push to a full mirror of all refs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -11,11 +11,13 @@ jobs:
     runs-on: eulernest
 
     steps:
-      - uses: actions/checkout@v4
-        with:
-          fetch-depth: 0   # vollständige History für mirror-push
-
-      - name: Push to Codeberg mirror
+      - name: Mirror all branches to Codeberg
+        env:
+          CODEBERG_TOKEN: ${{ secrets.CODEBERG_TOKEN }}
         run: |
-          git remote add codeberg https://TMoussa:${{ secrets.CODEBERG_TOKEN }}@codeberg.org/TMoussa/ConformalLabpp.git
-          git push codeberg HEAD:${{ github.ref_name }} --force
+          git clone --bare \
+            https://oauth2:${GITHUB_TOKEN}@git.eulernest.eu/conformallab/ConformalLabpp.git \
+            repo.git
+          cd repo.git
+          git push --mirror \
+            https://TMoussa:${CODEBERG_TOKEN}@codeberg.org/TMoussa/ConformalLabpp.git