ci: run Pull Request CI for PRs targeting release/** branches

Enables the full validate-and-build pipeline on PRs whose base is a
release branch (e.g. release/2.8.0), not just main, so milestone PRs
retargeted onto the release branch get per-PR CI before merge.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/pull-request.yml

@@ -2,7 +2,7 @@ name: Pull Request CI
 
 on:
   pull_request:
-    branches: [ main ]
+    branches: [ main, "release/**" ]
 
 permissions:
   contents: read