Allow users to select which organization's Copilot seat to use

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

As a GitHub user who is a member of multiple organizations (both providing Copilot seats), I have no way to control which organization's seat is used for my Copilot requests.

### Proposed solution

As a GitHub user who is a member of multiple organizations (both providing Copilot seats), I have no way to control which organization's seat is used for my Copilot requests.

Current behavior: GitHub automatically selects the organization seat based on repository context or alphabetical order — with no user-facing override.

Expected behavior: Provide a way to explicitly select the preferred organization seat, for example:

- A CLI command like /org or a config setting
- An environment variable (e.g. COPILOT_ORG=eviden)
- A setting in the Copilot CLI config file

Why this matters:

- Different organizations may have different Copilot policies, model access, or usage quotas
- Users working outside of any org repository context have no control over which seat is consumed
- Enterprise customers with multiple org memberships need transparency and control

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by reviewing the Copilot CLI configuration and current organization-seat selection behavior described in the issue. Compare the proposed command, environment variable, and config-file approaches; the work is done when users can explicitly choose which organization's seat handles their Copilot requests.

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