fix(ci): flatten RPM artifacts before signing

Gitea's download-artifact does not support merge-multiple, so RPMs
end up in subdirectories. Add a step to move them into the expected
flat directory before publish-repo.sh runs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-release.yml

@@ -119,7 +119,11 @@ jobs:
         with:
           path: rpms/
           pattern: rpm-*
-          merge-multiple: true
+
+      - name: Flatten RPM artifacts
+        run: |
+          find rpms/ -name '*.rpm' -exec mv --target-directory=rpms/ {} +
+          find rpms/ -mindepth 1 -type d -empty -delete
 
       - name: Import signing key
         run: |