`gh stack view` silently ignores positional arguments, including a valid different stack number

## Summary

`gh stack view` accepts no positional argument, but supplying one is silently discarded instead of rejected. The harmful case is passing a **valid stack number that isn't the current one** — you get a different stack's contents, with that stack's number in the header, and no indication your argument was ignored.

On a branch belonging to stack #6, asking for stack #3 (which exists):

```
$ gh stack view --short 3
Stack #6 <-- rendered #6, not the #3 that was requested
├ stack-b-two ○ #5
» stack-b-one ○ #4 (current)
└ main

$ gh stack view --short garbage999
Stack #6 <-- also silently succeeds
├ stack-b-two ○ #5
» stack-b-one ○ #4 (current)
└ main
```

Exit status is 0 in both cases.

## Why this matters

The output carries a stack number in its header, so it reads as authoritative. Anyone comparing two stacks — or scripting against `view` — can act on the wrong one without any signal. It is also a natural mistake to make, because `gh stack checkout` *does* take a stack number, so the two commands look like they share a selector.

## Reproduction

In a repo with two stacks:

```bash
git switch
gh stack view --short # shows stack B
gh stack view --short garbage999 # shows stack B
```

Public repo with two stacks set up for this: https://github.com/xn/gh-stack-repro

## Expected

Either error with `unknown argument "3"` (the usual `gh` behaviour for unexpected positionals), or accept a stack selector here the way `gh stack checkout` does. The latter would also give a non-interactive escape hatch for the ambiguity in #415.

## Environment

- `gh` 2.97.0
- `gh-stack` v0.1.0
- git 2.50.1
- macOS 26.4.1, darwin/arm64

`gh stack view` コマンドの引数解析から始め、issue にある 2 つのコマンドを、終了ステータスと出力を含めて再現してください。`gh stack checkout` が stack 番号をどのように処理するかを確認し、その後、有効および無効な位置引数のテストカバレッジを追加してください。完了の条件は、コマンドがそれらを暗黙的に無視しなくなることです。明確に拒否するか、stack の選択を一貫してサポートする必要があります。

索引モデルが issue の本文から書いたものです。