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

## 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`.

Bắt đầu với lệnh gh stack link và tái hiện lỗi bằng các bước trong issue này, sau đó kiểm tra các lệnh gọi ảnh hưởng đến tư cách thành viên của repos/{owner}/{repo}/stacks/{id}. Theo dõi cách một lần thử tạo PR thất bại xử lý các thay đổi tư cách thành viên trước đó và sau đó. Hoàn thành khi một liên kết thất bại либо giữ nguyên tư cách thành viên pull_requests ban đầu hoặc báo cáo rõ ràng mọi thay đổi từng phần, có xác minh với API.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.