fix: verify release artifacts exist before creating release

Add a check that artifacts were downloaded successfully before attempting
to create the release. This is equivalent to the `fail_on_unmatched_files: true`
option in the old `softprops/action-gh-release` action.

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

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

Author: kim-em

.github/workflows/pr-release.yml

@@ -72,9 +72,22 @@ jobs:
           gh release delete --repo ${{ github.repository_owner }}/lean4-pr-releases pr-release-${{ steps.workflow-info.outputs.pullRequestNumber }}-${{ env.SHORT_SHA }} -y || true
         env:
           GH_TOKEN: ${{ secrets.PR_RELEASES_TOKEN }}
+      # Verify artifacts were downloaded (equivalent to fail_on_unmatched_files in the old action).
+      - name: Verify release artifacts exist
+        if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
+        run: |
+          shopt -s nullglob
+          files=(artifacts/*/*)
+          if [ ${#files[@]} -eq 0 ]; then
+            echo "::error::No artifacts found matching artifacts/*/*"
+            exit 1
+          fi
+          echo "Found ${#files[@]} artifacts to upload:"
+          printf '%s\n' "${files[@]}"
       # We use `gh release create` instead of `softprops/action-gh-release` because
       # the latter enumerates all releases to check for existing ones, which fails
       # when the repository has more than 10000 releases (GitHub API pagination limit).
+      # Upstream fix: https://github.com/softprops/action-gh-release/pull/725
       - name: Release (short format)
         if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
         run: |