fix: convert to proper OpenCode Plugin function signature

- Export async Plugin function instead of object
- Add @opencode-ai/plugin dependency
- Implement auth hook with loader and OAuth methods
- Wire multi-account rotation into custom fetch

BREAKING: Plugin now follows the OpenCode plugin contract.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>