fix: custom endpoint model validation + better /model error messages

- Custom endpoints can serve any model, so skip validation for
  provider='custom' in validate_requested_model(). Previously it
  would reject any model name since there's no static catalog or
  live API to check against.
- Show clear setup instructions when switching to custom endpoint
  without OPENAI_BASE_URL/OPENAI_API_KEY configured.
- Added curated model lists for Nous Portal and OpenAI Codex to
  _PROVIDER_MODELS so /model shows their available models.

hermes_cli/models.py

@@ -276,6 +276,15 @@ def validate_requested_model(
             "message": "Model names cannot contain spaces.",
         }
 
+    # Custom endpoints can serve any model — skip validation
+    if normalized == "custom":
+        return {
+            "accepted": True,
+            "persist": True,
+            "recognized": False,
+            "message": None,
+        }
+
     # Probe the live API to check if the model actually exists
     api_models = fetch_api_models(api_key, base_url)