Show me want account will be used when running a prompt in non-interactive mode

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

When I'm logged into multiple accounts and I want to run Copilot CLI non-interactively, I want to know which account will be used for the requests so that I can expect when I'm billed.

### Proposed solution

A few ideas:

- If I'm logged into 1 account, just run the prompt as normal
- If I'm logged into multiple accounts, either
- prompt me for account selection when I try to run it
- support a new `--account` (or `--user`, whatever) argument + error when not passed in

If we don't want to support account selection (in either forms), I'd at least want the info to be shown as part of the output. For example:

```
% copilot -p "update all configs where \`team_slack\` is either #foo or #boor such that they're now #foo-feedback and #bar-feedback instead, respectively"

Running prompt as: mxie (via gh)

✓ Grep "team_slack\s*[:=]\s*#?foo" (./)
└ 5 lines found
```

### Example prompts or workflows

Anything along the lines of: `copilot -u -p "some prompt here"`

### Additional context

_No response_