Support per-session model override without persisting to settings

Problem

/model writes the selected model to settings.json, making it a global change across all sessions. If I switch to Sonnet in one terminal session for a quick task, every other running and future session also switches. There's no way to temporarily use a different model in a single session without affecting others.

Proposed solution

Add a session-only model override that doesn't persist to settings.json. Two complementary approaches:

1. CLI flag: claude --model sonnet — starts the session with a model override without writing to settings
2. Session-only /model: /model --session (or make session-only the default for /model, with /model --save to persist)

The override should take precedence over settings.json and project-level settings for the duration of that session only.

Current workaround

Manually switch back with /model after finishing, which is error-prone and still briefly affects other sessions.