Agent reasoning should respect user's language

What feature would you like to see?

Problem

Agent reasoning in TUI is always displayed in English, regardless of the user's input language. This breaks the narrative flow for non-English users.

Impact

• Cognitive load: Users must mentally switch between languages while reading reasoning + writing prompts
• Lost context: The reasoning is the primary way users understand agent decisions — if it's in a foreign language, this value is diminished
• Inconsistent UX: User writes in their language → agent reasons in English → creates jarring experience

Proposed solution

Add language detection for user input and instruct the model to output reasoning in the matched language.

Alternatively, expose a reasoning_language config option in config.toml:

[model]
reasoning_language = "auto"  # or "en", "ru", "zh", etc.

Prior art

Claude Code handles this gracefully — reasoning appears in the language the user communicates in.

Additional information

Tested on macOS with Codex CLI. User input in Russian, all reasoning output appeared in English.