Contributor nudge: CI-failure nudges churn on the same PR as the failing-check set and reason prefix flap #17328

What happens

On hive's own spoke (since this morning's roll) the stage-072 contributor nudge re-fires repeatedly on the same PR while CI keeps failing across pushes and reruns (#17303 / #17305 / #17308). Pod log excerpt for #17308, one head (60ed7e2), five minutes:

17:09 contributor nudged ... nudge=1 posted=true  mode=preempt reason=check-failing:changelog-fragment-guard
17:13 contributor nudged ... nudge=1 posted=false mode=queued  reason=optional-check-failing:native-queue-policy
17:14 contributor nudged ... nudge=1 posted=false mode=queued  reason=required-check-failing:changelog-fragment-guard

#17073 made the nudge one comment per PR + reason class, but a CI failure is three reason classes (check-failing, optional-check-failing, required-check-failing → slugs check-failing / optional-check-failing), so as the lane's reason prefix flaps between them every pass is a "new class": the comment is rewritten, the relay gets another fix-pr dispatch, and the per-class nudge count restarts (so the cap of 2 never holds). Each new red head is another relay dispatch too, with no window between them.

What should happen

Per-PR CI-failure nudge dedupe + cooldown, keyed on (repo, PR, failing-check set):

1. All CI-failure reasons are one check-failing class. Same failing set → the existing hive:contributor-nudge comment is edited in place with the new head and an attempt count; never a second comment.
2. On the same head the failing set only accumulates (union); another check going red folds into the comment without a relay re-dispatch.
3. A changed failing set on a new head within governor.labels.stage.contributor_nudge.ci_failure_cooldown (default 6h) folds into the existing comment as a rewrite (count unchanged); after the cooldown it counts as a fresh nudge.
4. State lives in the stage-label store (/data JSON) so a restart does not re-nudge.

Settings-UI parity for the new key (Labels tab, next to the re-nudge fields).