Fix signing configuration for Gradle Plugin Portal publishing

- Add signing.key and signing.password properties to publishPlugins step
- Ensures signing works for both Maven Central and Gradle Plugin Portal
- Fixes 'no configured signatory' error during plugin portal publication

Author: gamma

.github/workflows/release.yml

@@ -131,8 +131,12 @@ jobs:
         env:
           GRADLE_PUBLISH_KEY: ${{ secrets.GRADLE_PUBLISH_KEY }}
           GRADLE_PUBLISH_SECRET: ${{ secrets.GRADLE_PUBLISH_SECRET }}
+          SIGNING_KEY: ${{ secrets.SIGNING_KEY }}
+          SIGNING_PASSWORD: ${{ secrets.SIGNING_PASSWORD }}
         run: |
-          gradle publishPlugins --no-daemon
+          gradle publishPlugins --no-daemon \
+            -Psigning.key="$SIGNING_KEY" \
+            -Psigning.password="$SIGNING_PASSWORD"
       
       - name: Create GitHub Release
         uses: softprops/action-gh-release@v1