Feature Request: Add option to customize session title language

[togo01]

Summary

Currently, session titles are automatically generated in English by the small_model. It would be helpful to have an option to customize the language for session title generation.

Use Case

For non-English speaking teams or projects with localized documentation, having session titles in the local language (e.g., Traditional Chinese, Japanese, etc.) would improve readability and organization. I tried adding language instructions to AGENTS.md:

### Session 標題
當為對話產生標題時，**必須使用繁體中文**。

However, this has no effect since title generation appears to be an independent lightweight task that doesn't read the project's AGENTS.md instructions. Proposed Solution Add a configuration option in opencode.json, for example:

{
  $schema: https://opencode.ai/config.json,
  title_language: zh-TW
}

Or alternatively:

{
  $schema: https://opencode.ai/config.json,
  locale: zh-TW
}

Alternatives Considered

1. Adding instructions to AGENTS.md - doesn't work
2. Adding instructions to global ~/.config/opencode/AGENTS.md - doesn't work Additional Context

• The small_model is used for title generation according to the docs
• This would benefit users in non-English speaking regions