ci(autoupdate): also trigger on pull_request so newly-opened PRs update immediately

The workflow only fired on push to main, so a PR opened during a quiet
period (no subsequent merge to main) sat behind main until manually
updated. Add an opened/reopened/ready_for_review pull_request trigger;
synchronize is intentionally excluded to avoid re-running on every push,
including the autoupdate action's own branch updates.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/autoupdate.yml

@@ -17,6 +17,8 @@ name: Auto-update PR branches
 on:
   push:
     branches: [main]
+  pull_request:
+    types: [opened, reopened, ready_for_review]
 
 permissions:
   contents: write