[Feature Request] Add discoverable model list with offline fallback support

Bug Description
▎ Feature request: discoverable model list + outage-resilient fallback.
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▎ /model currently shows only the latest-generation models. Older models are still healthy and switchable, but only if you already know their exact ID string — there's no way to enumerate them from the picker.
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▎ This bites hardest during an outage: the moment you most need to fall back to a different model is exactly when you can't ask the assistant which models are available, because the model that would answer that question is the one that's down. The fallback path can't depend on the failing component.
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▎ Asks:
▎ - /model should list all selectable models, including older generations — not just the current flagship set.
▎ - Model enumeration should be static CLI data (no model round-trip required), so it works even when every model endpoint is failing.
▎ - Bonus: a live health indicator per model, so you can see which to switch to.

Environment Info

• Platform: win32
• Terminal: windows-terminal
• Version: 2.1.222
• Feedback ID: 1680bc85-90d8-4823-9000-bb1ddbb0e9c4

Errors

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