Stack panel says a pull request is queued when it has no queue entry

## Summary

On a stacked pull request that is not in the merge queue, the stack panel says it is, and offers to remove it. The badges in the same panel report the state correctly, so the panel contradicts itself.

## Evidence

Stack `main ← #101 ← #102`. After **Enqueue stack (2)**, only `#101` is admitted. The page of `#102` then shows:

- the heading **"Queued to merge…"**
- **"This pull request is next up in the merge queue."**
- a **"Remove from queue"** button
- badges `#102 Ready` and `#101 Queued`, which are correct

The merge queue page for the same branch shows **1 Queued** and lists only `#101`:

The API agrees with the queue page, not with the heading:

```text
mergeQueue(branch:"main").entries.totalCount = 1
mergeQueue(branch:"main").entries.nodes = [{ position: 1, pullRequest: #101 }]

#101 isInMergeQueue true position 1
#102 isInMergeQueue false mergeQueueEntry null
```

`#102` reported `isInMergeQueue=false` at every sample across the whole landing.

## Expected

A stacked pull request with no queue entry should not be described as queued:

- do not head the panel "Queued to merge…" on a pull request that is not queued;
- do not say "This pull request is next up in the merge queue" when `isInMergeQueue` is `false`. If the intent is that this layer goes after the one below it, say that. It is a statement about the stack, not about queue membership;
- do not offer "Remove from queue" for a pull request with no `mergeQueueEntry`. The button does not say whether it would remove the layer below, which would be destructive.

## Actual

The heading, the sentence and the button all claim queue membership that the API denies, while the badges in the same panel report it correctly.

## Environment

- `gh` 2.100.0, `gh stack` v0.0.8
- Trunk ruleset:
```
merge_queue: grouping_strategy HEADGREEN, merge_method REBASE,
max_entries_to_build 8, max_entries_to_merge 8,
min_entries_to_merge 1, min_entries_to_merge_wait_minutes 5,
check_response_timeout_minutes 40
pull_request: allowed_merge_methods ["rebase"], required_approving_review_count 1,
require_last_push_approval false, dismiss_stale_reviews_on_push false
```
- `delete_branch_on_merge: true`, `allow_auto_merge: false`

## Reproduction

1. Protect a trunk with a merge queue. Run `gh stack link ` and get both pull requests approved and green.
2. Press **Enqueue stack (2)**.
3. Open the top pull request and compare its panel against the branch's merge queue page and against `pullRequest{ isInMergeQueue mergeQueueEntry{ position } }`.

## Related

- #174 — why the upper layer is out of the queue in the first place
- #172 — "'Merge stack' appears to enqueue only the bottom PR"
- #502 — the wording matters most there, because the rewrite forces a manual re-enqueue and this panel gives no sign of it

Start with the stack panel state and the pullRequest query fields isInMergeQueue and mergeQueueEntry{ position }; reproduce the two-PR case by comparing the panel with the merge queue page and API response. Done means an unqueued upper pull request no longer gets queued wording or a Remove from queue action, while the existing correct badges remain correct.

Escrito por el modelo de indexación a partir del texto del issue.