Reject providers with no models into bad_providers and warn with reasons
on chat/providers. When /provider changes and the current model is not
listed, prompt for a new model (keep it when still supported).
Store provider_name/model on sessions (SQLite migrate); restore on
resume and persist on create//model//provider. Startup prefers session
selection over re-asking. ask/choose and provider/model pick use
readline with Tab completion when available.
NCBM