Fix release asset upload by checking out repository

The gh CLI needs to be run in a git repository to determine the target
repo for uploading release assets. Add checkout step to the
upload-to-release job.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -89,6 +89,9 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
       - name: Download all artifacts
         uses: actions/download-artifact@v4
         with: