A PR ejected from the merge queue by the needs-validation gate leaves no
trace on the PR itself: the gate fails on the queue's transient ref, the
PR's own checks stay green, and mergeQueueEntry silently goes null. The
only way to learn why the PR fell out of the queue is to dig through
merge_group workflow runs, which nobody watches (observed on #5329).
Producer: the merge_group gate in ci.yml now emits a handoff/comment
artifact targeting the labeled PR right before it blocks, and uploads it
on the failure path. Consumer: comment.atom.yml accepts merge_group
workflow_run events; since a merge_group run's head_sha is the queue's
synthetic merge commit, artifacts from those runs are bound to their
producing run by run_id, and the base-freshness skip is waived (PRs
ahead in the queue move the base while the run completes). The live-PR
head check still guards staleness.
Topology coverage pins the new boundary: comment.atom consumes
pull_request + merge_group; autofix/report stay pull_request-only.
lefarcen