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_

Nenhum arquivo ou teste é especificado. Comece localizando o parsing dos argumentos da CLI e o fluxo existente de seleção de conta por meio de `/user`; em seguida, rastreie como a conta selecionada chega a uma nova sessão. A tarefa estará concluída quando um startup flag selecionar de forma confiável a conta solicitada, enquanto o comportamento existente permanece inalterado e a opção é fácil de encontrar.

Escrita pelo modelo de indexação a partir do texto da issue.