feat: Add support for OpenCode Zen and Go

## Model Provider Integration Request

OpenCode Zen and Go. OpenCode Go is a cheap provider of open-source models, Zen provides a good reliablity and access to the models, you can think of them as a router that tries to give you the best possible inference.

I think it will also be useful for stuff like [boxxy](https://github.com/boxxy-dev/boxxy/issues/35)

### Resources

- [https://opencode.ai/zen](https://opencode.ai/zen)
- [https://opencode.ai/docs/zen/](https://opencode.ai/docs/zen/)
- [https://opencode.ai/go](https://opencode.ai/go)
- [https://opencode.ai/docs/go/](https://opencode.ai/docs/go/)

The issue requests adding support for OpenCode Zen and Go as model providers. Start by examining existing provider integrations in the codebase, likely under a directory like `src/providers/`. Review the provided documentation links to understand the APIs. Determine the required changes to the provider abstraction, authentication, and request/response handling. A successful integration would allow configuring and using these providers within the rig framework.

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