Fix the surprising auto-close of stacked / dependent PRs when their
base branch gets deleted on GitHub. Use when: (1) you ran
`gh pr merge --delete-branch` (or any branch-deleting merge action) on
PR A, and PR B which was based on PR A is now CLOSED with no warning —
not retargeted to the repo's default branch as you might expect,
(2) a stacked-PR workflow where each PR in the chain has its base set
to the previous PR's head, and you'd like to land them in sequence
without losing the dependent PRs' review history, (3) you want to
reopen a PR closed by the base-branch-delete cascade. Root cause:
GitHub auto-closes (does not retarget) dependent PRs when their base
ref vanis…
