Fix CI: use github.ref_name for branch in SSH command

GITHUB_REF wasn't available inside the remote SSH session.
Use Gitea's template interpolation instead.

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

.gitea/workflows/ci.yml

@@ -30,13 +30,13 @@ jobs:
       - name: Pull latest code on server
         env:
           DEPLOY_HOST: ${{ secrets.DEPLOY_HOST }}
+          BRANCH: ${{ github.ref_name }}
         run: |
           ssh "root@$DEPLOY_HOST" "
-            BRANCH="${GITHUB_REF##refs/heads/}"
             cd /root/art-dag/celery
-            git fetch origin "$BRANCH"
-            git checkout "$BRANCH"
-            git reset --hard "origin/$BRANCH"
+            git fetch origin $BRANCH
+            git checkout $BRANCH
+            git reset --hard origin/$BRANCH
           "
 
       - name: Build and push image