unstack shows a generic/incorrect error instead of the real API rejection reason

Start by reproducing gh stack unstack <number> on a stack with merged pull requests, comparing normal output with GH_DEBUG=api. Trace the unstack error handling and the REST API's 422 response; done means the non-debug output reports the actual rejection reason rather than the unrelated queued-for-merge message.

Summary

gh stack unstack <number> fails on a stack containing merged PRs, but the plain (non-debug) error message names an unrelated cause. The actual server-side reason is only visible with GH_DEBUG=api.

Repro

1. Create a stack with gh stack link (or submit), e.g. main <- a <- b <- c <- d <- e.
2. Merge a and b (via merge queue or a normal merge — doesn't matter which).
3. Run gh stack unstack <number> on that stack.

Actual

⚠ Some pull requests are queued for merge or have auto-merge enabled and remain stacked on GitHub
The stack was left in place — local tracking is unchanged

I verified via gh api graphql that none of the stack's PRs had autoMergeRequest set or a mergeQueueEntry — so the stated reason doesn't match reality.

Running the identical command with GH_DEBUG=api gh stack unstack <number> shows the real HTTP exchange:

POST /repos/{owner}/{repo}/stacks/{id}/unstack
< HTTP/2.0 422 Unprocessable Entity
{
  "message": "Pull requests #.. , #.. cannot be removed from this stack",
  "documentation_url": "https://docs.github.com/rest/pulls/stacks#remove-pull-requests-from-a-pull-request-stack",
  "status": "422"
}

Interestingly, in the GH_DEBUG=api run the CLI's own final stderr line correctly echoed this specific message (✗ Unstacking not allowed: Pull requests #.., #.. cannot be removed from this stack) — a materially different, and accurate, explanation compared to the generic message shown without debug logging.

Expected

The non-debug error message should reflect the actual REST API rejection (a stack can't be unstacked once any of its PRs have merged), not a generic, unrelated "queued for merge / auto-merge enabled" message.

Environment

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