Add GitHub Copilot as a fallback provider option in the installer. Native providers (Claude/ChatGPT/Gemini) have priority over Copilot.
- Add hasCopilot field to InstallConfig and DetectedConfig types
- Add --copilot flag support for both TUI and non-TUI modes
- Add Copilot prompt in interactive installer (after Gemini)
- Update model selection logic to use Copilot fallback when native providers unavailable
- Update configuration summary to display Copilot status
Addresses #762
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>