One-PR stack: submit reports success, but merge says the stack was not submitted

## Summary

A local stack can contain one branch and one pull request. `gh stack submit` reports success for this stack:

```text
✓ Pushed and synced 1 branches
```

However, GitHub does not create a remote stack object. A remote stack requires at least two pull requests.

`gh stack merge` then rejects the local stack:

```text
✗ this stack has not been submitted to GitHub yet; run `gh stack submit` first
```

A second `gh stack submit` does not fix the problem. It reports success again.

## Reproduction

1. Create a branch from the trunk branch.
2. Commit a change on the new branch.
3. Initialize a local stack that contains only the new branch.
4. Run `gh stack submit` and create the pull request.
5. Run `gh stack submit` again.
6. Run `gh stack merge --squash`.

Example output:

```text
$ gh stack submit
Checking stack state...
Pushing to origin...
PR #30 for close-clinkr-cli-entry-point is up to date
✓ Pushed and synced 1 branches

$ gh stack sync
✓ Fetched latest main from origin
✓ Trunk main is already up to date

Pushing 1 branch to origin...
✓ Pushed 1 branches

Syncing PRs ...
✓ PR #30 (close-clinkr-cli-entry-point) — Open

✓ Branches synced
Stacked on main (69c442c)

$ gh stack merge --squash
✗ this stack has not been submitted to GitHub yet; run `gh stack submit` first
```

The pull request exists and is open. In this reproduction, GitHub also reported the pull request as mergeable and clean.

## Expected behavior

The commands should not give conflicting results.

`gh stack submit` should state that it published a regular pull request but did not create a remote stack object. Alternatively, it should explain that a remote stack requires at least two pull requests.

`gh stack merge` should handle a local stack with one pull request. It can merge the pull request directly, or it can show an actionable command such as `gh pr merge`.

## Actual behavior

`gh stack submit` reports that it pushed and synchronized the one-branch stack. It exits successfully.

`gh stack merge` reports that the same stack was not submitted. It tells the user to repeat the command that already reported success.

## Source context

The behavior is also present on the current `github/gh-stack` `main` branch at commit `ab00aa4a3f2dddc51aa65849c68b391a1b079311`.

In `cmd/submit.go`, `syncStack` returns `false` when the stack has fewer than two pull requests:

```go
// The API requires at least 2 PRs to form a stack.
if len(prNumbers) < 2 {
return false
}
```

The caller does not use this result:

```go
if stacksAvailable {
syncStack(cfg, client, s)
clearPendingModifyState(cfg, gitDir)
}
```

The command then prints the success message:

```go
cfg.Successf("Pushed and synced %d branches", len(s.ActiveBranches()))
```

In `cmd/merge.go`, the merge command requires a remote stack ID or number:

```go
if s.ID == "" && s.Number == 0 {
cfg.Errorf("this stack has not been submitted to GitHub yet; run `gh stack submit` first")
return nil, ErrNotInStack
}
```

Thus, the submit command knows that it did not create a remote stack object. However, it discards that result and reports success.

## Environment

```text
gh stack version 0.1.0
Darwin 25.6.0 arm64
```

---
Deviations: none.

Bắt đầu trong cmd/submit.go bằng cách đọc syncStack và hàm gọi nó, sau đó so sánh kiểm tra remote-stack trong cmd/merge.go. Tái hiện luồng one-PR bằng các lệnh trong issue; hoàn tất có nghĩa là submit báo cáo chính xác remote-stack bị thiếu và merge đưa ra kết quả có thể sử dụng hoặc lệnh tiếp theo mà không yêu cầu người dùng lặp lại một lần gửi đã thành công.

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