feat: integrate gemini through the same login flow as gemini cli

[mgpai22]

Summary

• Adds Gemini CLI OAuth login flow integration
• Implements PKCE-based OAuth for secure browser authentication
• Handles token exchange, refresh, and caching automatically
• Supports both interactive and headless authentication modes

Fixes #8035

Test plan

• Test OAuth flow opens browser and completes authentication
• Verify token refresh works after expiry
• Confirm API requests succeed with obtained credentials
• Test headless mode fallback

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[mgpai22]

@codex review

[jenslys]

Is this not a 1:1 copy of my plugin?

https://github.com/jenslys/opencode-gemini-auth

[YaKun9]

这不是我插件的1：1复制品吗？

https://github.com/jenslys/opencode-gemini-auth

A very nice plugin — already using it and loving it.