bug(tui): model candidates come from different sources in the picker and wizard #248

Summary

Model candidates come from different places depending on where you are, so the
same provider offers different models in the TUI picker and in the wizard.

Detail

• The TUI picker lists the models written into the provider's config section,
  then a curated shortlist compiled into the package.
• The onboarding wizard fetches GET /v1/models from the provider and offers
  what it returns, falling back to the registry's default model.

Neither is wrong on its own. Together they mean a model offered in one place may
be absent from the other, and a stale shortlist is invisible next to a live
fetch.

Proposal

Decide one order of precedence and use it in both -- for example the user's
configured models, then a live fetch when credentials allow one, then the
shortlist -- so the two views cannot disagree. The shortlist's role should be
explicit: an offline starting point, not a competing catalogue.