Feature: Support Gemini CLI for PR code review

[Svtter]

Summary

Google's Gemini CLI supports non-interactive mode and has an official GitHub Action. It would be great if opencode-actions also supported Gemini CLI as a code review backend.

Motivation

• Gemini CLI is open source, actively maintained by Google
• Free tier available (60 RPM, 1000 RPD via OAuth; 1000 RPD via API key)
• Non-interactive mode is script-friendly
• Official GitHub Action exists for reference
• Multiple auth methods: OAuth, API Key, Vertex AI, WIF

Suggested Implementation

1. Add a new provider option (e.g. provider: gemini-cli)
2. Support existing Gemini auth methods (GEMINI_API_KEY, WIF)
3. Leverage non-interactive mode for review output
4. Post review comments back to PR, consistent with existing provider behavior

References

• google-gemini/gemini-cli
• google-github-actions/run-gemini-cli
• Google AI Studio