[Feature Request] Model picker lacks session-only switching option

Bug Description
The /model picker has no way to change model for the current session only — every selection is written as the new default for all future sessions ("Your pick becomes the default for new sessions"). This is backwards for my workflow: ~99% of the time I switch models I want it for just that one session, and only rarely do I want to move my persistent baseline.

The only current workaround is quitting and relaunching with --model, which is disruptive mid-session. Requests, any of which would fix this:

1. Add a session-only choice in the /model picker (e.g. a separate keybinding or a "just this session" row) alongside the save-as-default one.
2. Add a setting (e.g. modelPickerDefaultsToSession: true) that inverts the behavior so the picker changes the session only unless I explicitly opt to persist.
3. At minimum, make persisting an explicit confirmation rather than the automatic outcome.

Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.206
• Feedback ID: 9724d9fb-e275-4a8d-88b6-a59a45252b11

Errors

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