[FEATURE]: Expose the current TUI model to plugins #42817

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

TUI plugins can render beside the prompt, but they cannot read the model currently selected for the next prompt. Session messages and model events only expose submitted or durable state, so plugin UI can remain stale after the user changes the model picker.

Could api.state expose a read-only accessor such as:

api.state.model.current():
  | { providerID: string; modelID: string }
  | undefined

This would project the same live Solid state used by the model picker and prompt submission. Reads inside plugin components or memos would update when the selection changes; reads outside a reactive scope would be point-in-time. It would return undefined until a valid model is available.

This would support model-aware prompt UI, context indicators, and provider-specific controls without switching or persisting the model.

I'm looking for design feedback on the API placement, return shape, and reactive contract before preparing a PR.