fix: save API key to model config for custom endpoints (#4182)

Custom cloud endpoints (Together.ai, RunPod, Groq, etc.) lost their
API key after #4165 removed OPENAI_API_KEY .env saves.  The key was
only saved to the custom_providers list which is unreachable at
runtime for plain 'custom' provider resolution.

Save model.api_key to config.yaml alongside model.provider and
model.base_url in all three custom endpoint code paths:
- _model_flow_custom (new endpoint with model name)
- _model_flow_custom (new endpoint without model name)
- _model_flow_named_custom (switching to a saved endpoint)

The runtime resolver already reads model.api_key (runtime_provider.py
line 224-228), so the key is picked up automatically.  Each custom
endpoint carries its own key in config — no shared OPENAI_API_KEY
env var needed.

hermes_cli/main.py

@@ -1288,6 +1288,8 @@ def _model_flow_custom(config):
             cfg["model"] = model
         model["provider"] = "custom"
         model["base_url"] = effective_url
+        if effective_key:
+            model["api_key"] = effective_key
         model.pop("api_mode", None)  # let runtime auto-detect from URL
         save_config(cfg)
         deactivate_provider()
@@ -1309,6 +1311,8 @@ def _model_flow_custom(config):
             _caller_model = {"default": _caller_model} if _caller_model else {}
         _caller_model["provider"] = "custom"
         _caller_model["base_url"] = effective_url
+        if effective_key:
+            _caller_model["api_key"] = effective_key
         _caller_model.pop("api_mode", None)
         config["model"] = _caller_model
         print("Endpoint saved. Use `/model` in chat or `hermes model` to set a model.")
@@ -1460,6 +1464,8 @@ def _model_flow_named_custom(config, provider_info):
             cfg["model"] = model
         model["provider"] = "custom"
         model["base_url"] = base_url
+        if api_key:
+            model["api_key"] = api_key
         save_config(cfg)
         deactivate_provider()
 
@@ -1531,6 +1537,8 @@ def _model_flow_named_custom(config, provider_info):
         cfg["model"] = model
     model["provider"] = "custom"
     model["base_url"] = base_url
+    if api_key:
+        model["api_key"] = api_key
     save_config(cfg)
     deactivate_provider()