gpt-5-mini is not persistent set for future sessions

### Describe the bug

Model selection not persisted: after running `/model gpt-5-mini`, the selection is active for the
current session, but after closing and reopening the Copilot CL,I the model reverts to `claude-sonnet-4.6` instead of
persisting "gpt-5-mini".has reverted; expected behavior is that the chosen model persists across sessions; it occurs every time.

### Affected version

GitHub Copilot CLI 0.0.422.

### Steps to reproduce the behavior

(1) run "/model gpt-5-mini"
(2) close the terminal
(3) reopen Copilot

and observe that the model is changed and not `gpt-5-mini`any longer

### Expected behavior

If the user previously selected the gpt-5-mini model using the coman /model command, then for all future Copilot CLI sessions, gpt-5-mini should be set as the default model unless changed again. This ensures continuity in model selection based on the user's last explicit choice.

### Additional context

_No response_