fix: model_display_name ignores display_name for kimi-for-coding

Describe the bug
The Kimi Code backend returns the correct display_name (e.g. "Kimi-k2.6") for the "kimi-for-coding" model via the /models endpoint, and this value is correctly saved to the config file. However, model_display_name() in src/kimi_cli/llm.py contains a hardcoded override that forces any model named "kimi-for-coding" or "kimi-code" to display as "kimi-for-coding" instead of respecting the backend-provided display_name.

This causes the welcome panel, prompt status bar, and other UI elements to show the opaque internal ID instead of the human-readable model name (e.g. "Kimi-k2.6").

To Reproduce

1. Log in via /login with Kimi Code platform.
2. Run kimi and observe the welcome panel or prompt status bar shows "kimi-for-coding" instead of "Kimi-k2.6".
3. Query the API directly: curl -H 'Authorization: Bearer <token>' https://api.kimi.com/coding/v1/models — the response includes display_name: 'Kimi-k2.6'.
4. Check ~/.kimi/config.toml — display_name = 'Kimi-k2.6' is correctly persisted, but never shown.

Expected behavior
The UI should show "Kimi-k2.6" (or whatever display_name the backend provides) for the kimi-for-coding model.

Environment

• kimi-cli version: 1.41.0
• Platform: Kimi Code (OAuth)

Start in src/kimi_cli/llm.py at model_display_name() and trace how the saved display_name from ~/.kimi/config.toml reaches the UI. Verify the welcome panel and prompt status bar show the backend-provided name, such as “Kimi-k2.6”, for the kimi-for-coding model rather than its internal ID.