Replace custom device-flow OAuth and token exchange with the official
@github/copilot-sdk. The SDK manages a Copilot CLI subprocess for
authentication and API access.
Changes:
- Add @github/copilot-sdk dependency (v0.1.19)
- Create github-copilot-sdk.ts as core SDK wrapper
- Rename github-copilot-auth.ts to github-copilot-login.ts
- Merge github-copilot-token.ts into github-copilot-sdk.ts
- Remove extensions/copilot-proxy plugin entirely
- Update login flow to verify CLI auth status instead of device flow
- Add SDK-based model discovery via listCopilotModels()
- Update docs and UI labels to reflect SDK-based authentication
- Remove copilot-proxy from auth choices and plugin auto-enable
BREAKING CHANGE: GitHub Copilot now requires the Copilot CLI to be
installed and authenticated via 'copilot auth login' before use.
