Integration of GitHub Copilot CLI as a first-class model provider within OpenWiki.

## Feature Request

Add support for **GitHub Copilot CLI** as an inference provider in OpenWiki.

### Motivation

Many enterprise users already have access to GitHub Copilot through their organization's licensing and authentication flows. Supporting GitHub Copilot CLI would allow OpenWiki users to generate and maintain repository documentation without provisioning separate API keys for OpenRouter, OpenAI, Anthropic, Fireworks, or Baseten.

This would reduce onboarding friction and enable teams to leverage existing GitHub Copilot subscriptions for OpenWiki workflows.

### Proposed User Experience

During initialization:

```bash
openwiki --init
```

Users should be able to select:

```text
Select model provider:

○ OpenRouter
○ OpenAI
○ Anthropic
○ Fireworks
○ Baseten
○ GitHub Copilot CLI
```

OpenWiki would then:

1. Detect whether GitHub Copilot CLI is installed.
2. Verify authentication status.
3. Enumerate available Copilot models.
4. Allow the user to choose a default model.
5. Persist the provider configuration in `~/.openwiki/.env`.

Example configuration:

```env
OPENWIKI_PROVIDER=copilot
OPENWIKI_MODEL=gpt-5
```

### Potential Implementation

GitHub Copilot CLI already provides:

* Authentication management
* Model selection
* Access to multiple foundation models
* Enterprise governance and licensing

OpenWiki could introduce a provider adapter similar to the existing provider abstractions and route inference requests through the locally authenticated Copilot CLI ses...