Add `--user` to copilot argument

### Describe the feature or problem you'd like to solve

I'd like to be able to specify which user I want to use for the session as I have both work and personal accounts

### Proposed solution

If you have a script that puts in all the startup values you like by default, would be nice to have an ability to also specify the user/account that the session should start up in. Kind of annoying to have to later specify the user via `/user`.

### Example prompts or workflows

I have a powershell alias `cs` that launches copilot with a few args like `--yolo` and a few other fields. I'd like to also be able to pass a flag like `-Personal` or `-Work` to force it to use a specific account for the new session (so I don't forget and spend personal on work or vice versa). Just relying on the last one launched gets tricky because sometimes I forget and in the morning when I get to work and I forget to switch back to work account.

### Additional context

_No response_

ファイルやテストは指定されていません。まず CLI の引数解析と、既存の `/user` によるアカウント選択フローの場所を特定し、次に選択されたアカウントが新しいセッションにどのように渡されるかを追跡してください。既存の動作を変更せず、startup flag で要求されたアカウントを確実に選択でき、かつそのオプションを見つけられる状態になれば完了です。

索引モデルが issue の本文から書いたものです。