Add GitHub Copilot CLI provider support

[shakecodeslikecray]

Description

Add support for GitHub Copilot CLI (gh copilot) as an LLM provider.

Tasks

• Research gh copilot capabilities
• Create src/providers/adapters/copilot.ts
• Add detection in src/providers/detect.ts
• Add tests
• Update documentation

Acceptance Criteria

• Users with gh copilot can use it as a provider
• Provider detection works correctly