[Feature Request] Add session-scoped model selection without persisting global default

Bug Description
Right now /model always persists the choice as the global default for new sessions. There's no way to change the model for just the current session from inside the session — the only session-scoped option is the --model flag at launch, which doesn't help once you're already running.

This breaks a common multi-terminal workflow. I often want different models in different sessions at the same time — e.g. Fable for orchestration in one terminal, Sonnet for writing tests in another. But because /model writes globally, the moment I switch one session and then open new terminals, they all inherit whatever I picked last. I end up with Sonnet orchestrating and Fable grinding through an implementation that should have gone to a faster model — and I don't notice until I'm deep into the session.

What I'd want: the model choice made via /model stays scoped to the session that made it, and doesn't rewrite the global default. Either as the default behavior, or as an explicit option in the picker (e.g. "this session only" vs. "set as default"), or a flag like /model sonnet --session.

The core issue is that a per-session setting is being stored as global state. Session-local memory for this would cost nothing and would fix the whole workflow.

Environment Info

• Platform: darwin
• Terminal: tmux
• Version: 2.1.228
• Feedback ID: c9296b55-0fcb-47f0-a090-1dd9a66df0e0