Fix CI: rename secret to REGISTRY_TOKEN (GITEA_ prefix reserved)

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

.gitea/workflows/build.yaml

@@ -39,7 +39,7 @@ jobs:
         run: |
           docker login ${{ env.REGISTRY }} \
             -u ${{ gitea.actor }} \
-            -p ${{ secrets.GITEA_TOKEN }}
+            -p ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Push image
         if: gitea.event_name != 'pull_request'