A failed link call leaves the target stack's membership partially mutated

从 gh stack link 命令开始，按照此 issue 中的步骤重现失败，然后检查影响 repos/{owner}/{repo}/stacks/{id} 成员关系的调用。跟踪 PR 创建尝试失败时如何处理之前和之后的成员关系变更。完成标准是：失败的链接要么保留原始的 pull_requests 成员关系，要么明确报告每一项部分变更，并通过 API 进行验证。

Summary

When gh stack link fails partway through (see #373 for one way to trigger this), the stack's membership on GitHub is left altered rather than restored — PRs that were part of the stack before the failed call are silently dropped from it, with no rollback and no mention of this in the command's error output.

Repro

1. Build a stack main <- a <- b <- c <- d <- e via gh stack link. Confirm via gh api repos/{owner}/{repo}/stacks/{id} that all 5 PRs are listed under pull_requests.
2. Merge a and b. Close c without merging.
3. Run gh stack link a b c d e --base main to fix d's base (see #373) — it fails while trying to create a duplicate PR for a.
4. Re-check the stack's membership: gh api repos/{owner}/{repo}/stacks/{id} --jq '.pull_requests[].number'.

Actual

Before step 3, the stack listed all 5 PRs. After the failed link call in step 3, the stack's pull_requests field lists only the merged PRs (a, b) — c, d, and e are gone from the stack entirely, even though d and e were never touched by the failed create attempt (only the lookup step "found" them, per #373's output).

This is also visible in the GitHub UI: PRs that previously showed a "Part of a stack" banner no longer show it at all.

I did not attempt to trigger this deliberately — it was an unwanted side effect discovered while working around #373 and #372.

Expected

Either:

• The whole link operation should be atomic: a failure partway through (e.g. an individual PR-creation error) should not leave the stack's membership altered from before the call, or
• If partial application is intentional/unavoidable, the command should clearly report which stack-membership changes actually took effect before it errored out, so the caller can reconcile the difference instead of discovering it later via the API or UI.

Environment

gh-stack version 0.0.8, installed via gh extension install github/gh-stack.