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_

Es werden keine Dateien oder Tests genannt. Beginne damit, die CLI-Argumentanalyse und den bestehenden Ablauf zur Kontenauswahl über `/user` zu finden; verfolge anschließend, wie das ausgewählte Konto eine neue Sitzung erreicht. Erledigt ist die Aufgabe, wenn ein Startup-Flag zuverlässig das angeforderte Konto auswählt, während das bestehende Verhalten unverändert bleibt und die Option auffindbar ist.

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