A stacked pull request cannot use auto-merge

## Summary

`gh stack link` refuses a pull request that has auto-merge enabled, so adopting a stack costs a capability that an unstacked pull request keeps. Every layer then needs a person at the keyboard at the moment the layer below lands.

## Evidence

Both pull requests approved and green. `#101` had auto-merge enabled, so linking failed:

```text
Checking existing stacks...
Looking up PRs for 2 branches...
✗ PR #101 cannot be added to a stack: it has auto-merge enabled
```

Turning auto-merge off let the link succeed.

This compounds the behaviour in #174. When the bottom layer merges, the layer above leaves the queue and GitHub rewrites its head, which restarts its required checks. Nothing puts it back. With auto-merge unavailable, only a person can, and only once the restarted checks finish. A stack of three with a 15-minute required check needs three waits and three manual enqueues to land one change.

## Expected

A stacked pull request can use auto-merge, so each layer enqueues itself once its base merges and its checks pass.

If the two genuinely cannot coexist, say so where the user meets it: `gh stack link` should explain why auto-merge and stacks are exclusive, and the stack panel should say that each layer needs a manual enqueue.

## Actual

`gh stack link` rejects the pull request with no explanation of the conflict, and offers no alternative.

## Environment

- `gh` 2.100.0, `gh stack` v0.0.8
- Trunk protected by a merge queue, `merge_method: REBASE`
- Two-layer stack, both pull requests approved with required checks passing

## Reproduction

1. Open two pull requests that stack on each other, both approved and green.
2. Enable auto-merge on the bottom one.
3. Run `gh stack link `.

## Note on scope

This reports only what was observed: `gh stack link` rejects a pull request that already has auto-merge enabled. The reverse direction, enabling auto-merge on a pull request that is already stacked, was not tested, because the repository in question has `allow_auto_merge: false` set at the repository level for unrelated reasons.

## Related

- #174 — the layer above leaves the queue and is not put back, which is what makes the missing auto-merge costly
- #172 — "'Merge stack' appears to enqueue only the bottom PR"
- #502 — the merge queue rewrites a commit that needed no rewrite, which is what restarts the checks
- #503 — the panel gives no sign that a manual enqueue is needed

Start with the `gh stack link` command and reproduce the rejection using the two-layer scenario in the issue. Read the handling of auto-merge and stack linking alongside related issues #174, #172, #502, and #503. Done means stacked pull requests can retain auto-merge, or the command and stack panel clearly explain the incompatibility and manual enqueue requirement.

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