Stacked PRs merged out of order can strand the top PR's commits

With a GitHub stack (PR B based on PR A's branch), if A is merged into main first and B is then merged a few seconds later, B merges into A's now-dead branch, not into main. GitHub shows both as MERGED, but B's commits are missing from main. After any stack merge, verify with git merge-base --is-ancestor <B's head> origin/main. If it fails, cherry-pick B's commits onto a new branch off main and open a re-land PR. Related trap: deleting a branch that is another open PR's base (e.g. gh pr close --delete-branch on the lower PR) auto-closes the upper PR. To recover, push the base branch back, gh pr reopen, retarget it with gh pr edit --base main, and only then delete the old base.