Stack merge updated the base branch without a push event after a failed stack rebase

[apparelmagic-heath]

Repo apparelmagic/apparelmagic (private), stack #1199: 9 PRs, #1185 → … → #1240 → #1260.

1. #1260 was opened against main, then retargeted onto #1240's branch and added to the stack (2026-10-04 13:40 UTC). GitHub rebased the branch onto the stack tip (new commit bbeff03c22, committed by GitHub), but the PR's head stayed on the pre-rebase commit 1579b1c9f1. The stack showed "rebase failed" and wouldn't merge #1260. Force-pushing an identical commit (7954f08001) resynced the PR head.
2. The whole stack then merged at 13:47:04 UTC as 577accc411 on main (parents 83650cf7e1 and 7954f08001). main points at that commit, but:
   • no push event to refs/heads/main appears in the repo events API;
   • the repo activity API (ref=main) has no entry for that update, while the branch deletions two minutes later are logged;
   • push-triggered integrations didn't fire: AWS CodePipeline through the GitHub connection, and Vercel. There are no statuses on 577accc411.
3. Earlier stack merges on the same repo (66d0a50541 on 2026-09-29, 79a0ed4bd4 on 2026-10-01) logged as pr_merge, sent their push events, and triggered both integrations. The difference this time was the failed rebase in step 1 just before the merge.

Expected: a stack merge updates the base branch through the normal push path (activity log entry, push webhooks), even after a stack rebase failed and was repaired.

Workaround: merging a follow-up PR into main to send a push event.