Chose billing account from the CLI

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

When users have multiple billing accounts on their GitHub account (example is if two organisations they're part of have a Copilot plan), they must go to the website to change which organisation gets billed

### Proposed solution

It would be great to have the choice of changing the billing account from the CLI.

I would imagine a `/billing` menu that would then display which organisations are available in the logged-in account.
The chosen organisation would get billed for the session's usage.

Even better : a way to store, per-project, which organisation gets billed (maybe in an optional repo-specific file, or in the computer's Copilot config ?)

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start at the CLI's command or menu entry point and investigate how the logged-in account's available organisations and billing state are represented. Clarify whether the first deliverable is an interactive /billing choice, per-project persistence, or both; done means selecting an organisation changes which account is billed for the session.

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