review-checklist: update checklist on reviewer add/remove

Adding or removing a reviewer did not trigger a workflow run, leaving
the checklist comment stale until the next push.

Add review_requested and review_request_removed to the pull_request_target
activity types so the checklist updates immediately when a reviewer is
manually added or removed.

Author: brtnfld

.github/workflows/review-checklist.yml

@@ -23,7 +23,7 @@ on:
   # Safe: checkout has no ref: override so the base branch (develop) is always
   # used — the fork's code is never checked out or executed.
   pull_request_target:
-    types: [opened, synchronize, reopened]
+    types: [opened, synchronize, reopened, review_requested, review_request_removed]
     branches: [develop]
   workflow_run:
     workflows: ["Review Checklist (gather)"]