Two issues with /model preventing proper provider switching:
1. Bare provider names not detected: typing '/model nous' treated 'nous'
as a model name instead of triggering a provider switch. Fixed by adding
step 0 in detect_provider_for_model() that checks if the input matches
a known provider name/alias (excluding 'custom'/'openrouter' which need
explicit model names) and returns that provider's default model.
2. Custom endpoint details hidden: /model (no args) showed '[custom]' with
just a usage hint but no endpoint URL or model name. Now displays the
configured base_url for custom providers in both CLI and gateway.
Note: config base_url and OPENAI_BASE_URL are intentionally NOT cleared on
provider switch — dedicated provider paths (nous, anthropic, codex) have
their own credential resolution that ignores these, and clearing them would
destroy the user's custom endpoint config, preventing switching back.
Co-authored-by: Test <test@test.com>
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