Expand pull request action triggers for review mode

Currently review mode only triggers when a pull request is opened, synced, or reopened: https://github.com/anthropics/claude-code-action/blob/15db2b3c79c0681556c056e9bc3f61fd3fc0347d/src/modes/review/index.ts#L42

It would be great if allowed actions were expanded to more natural assignment:
- labeled
- review_requested

review_requested is maybe a bit more questionable since it requires a user, but could work similar to `assigneee_triggerr`.

However the actions already support `label_trigger` but it just gets bypassed with the early check as far as I can tell.

The use case is to assign only certain reviews for claude and only do it when the PR is ready. We can achieve that through a PR comment, but label is more natural and has the advantage of being trackable and filterable.