fix(ci): surface rpm --addsign error output in CI logs

Capture stderr from rpm --addsign so the actual gpg error is visible
when signing fails.

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

script/publish-repo.sh

@@ -9,7 +9,10 @@ REMOTE_DIR="/var/www/rpm/fedora/${FEDORA_VERSION}/x86_64"
 # sign each rpm with the imported gpg key
 for rpm in "${RPM_DIR}"/*.rpm; do
     echo "signing ${rpm}..."
-    rpm --addsign "${rpm}"
+    if ! rpm --addsign "${rpm}" 2>&1; then
+        echo "failed to sign ${rpm}" >&2
+        exit 1
+    fi
 done
 
 install --directory --mode 700 ~/.ssh