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_

No files or tests are named. Start by locating the CLI argument parsing and the existing `/user` account-selection flow; then trace how the selected account reaches a new session. Done means a startup flag reliably selects the requested account, while existing behavior remains unchanged and the option is discoverable.

Written by the indexing model from the issue text.