Feature: /model command should not auto-persist to default settings

Problem

When using /model <name> to temporarily switch models in a session, Claude Code automatically persists that choice to ~/.claude/settings.json as the new default. This breaks the mental model that a session-scoped command should only affect the current session.

Current behavior

1. Set default model in settings.json to Opus
2. Use /model haiku to test with Haiku for one session
3. Check settings.json → now shows Haiku as default (unwanted side effect)

Desired behavior

/model <name> should only change the model for the current session and NOT write to settings.json. Users should have a separate way to change their persistent default (e.g., /config model opus or a dedicated --set-default-model flag).

Workaround

Currently, users must use environment variables instead to avoid auto-persistence:

export CLAUDE_CODE_MODEL=opus

This prevents /model from persisting changes, but it's not intuitive.