C:/Program Files/Git/model <name> should ask before saving as default, and accept an explicit persistence argument

Version 2.1.220. Documented behavior today: typing /model <name> directly behaves like Enter in the picker — it switches and saves as your default for new sessions, with a confirmation line after the fact. The picker offers s for session-only, but the direct-typed form has no equivalent — the persistence decision is made for you and the notice arrives only after settings.json is already written.

Two changes, complementary:

1. Ask at switch time. /model fable with no further argument should prompt: this session only, or save as default? Today's after-the-fact notice is easy to miss mid-work (mine scrolled by; I found the changed default a day later via token spend on a premium model).
2. Explicit persistence argument, for scripting and certainty: /model fable --session and /model fable --persist (bikeshed the spelling) — whichever is stated skips the prompt. This also gives headless/automation a deterministic form.

Context: /model semantics already flipped once (v2.1.144–152 was session-only, then reverted). An explicit ask + explicit argument ends the ambiguity instead of picking a winner: users who want stickiness say so, users borrowing a premium model for one task don't inherit it globally.