When `fetch_nous_models()` fails silently during setup, the model
selection falls through to the OpenRouter static list. Users then pick
models in OpenRouter format (e.g. `anthropic/claude-opus-4.6`) which
the Nous inference API rejects with a 400 "missing model" error.
Add an explicit `elif selected_provider == "nous"` branch that prompts
for manual model entry instead of falling through to the generic
OpenRouter fallback.
8bf28e1441