fix(ci): Stop running changelog-check.yml on merge_group trigger

[rekmarks]

Removes the merge_group trigger from the changelog workflow in CI, which the action is not configured to support and will cause it to fail.

---

Note

Low Risk
Low risk workflow change limited to CI triggers; it only reduces when the job runs and should not affect runtime code.

Overview
Stops .github/workflows/changelog-check.yml from running on the merge_group event, leaving it triggered only by pull_request activity.

This avoids running the changelog check action in a context it isn’t configured to handle (merge queue), preventing CI failures there.

^{Reviewed by Cursor Bugbot for commit 55737d8. Bugbot is set up for automated code reviews on this repo. Configure here.}