Trigger CI with the labels attached

The workflow's pull_request trigger doesn't fire on `labeled`, so the
label-gated jobs need a synchronize event to be evaluated against.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UkNbgdyuKgHaFwT27RahGH

Author: kraenhansen