fix(config): preserve scalar model id when setting model sub-keys

When model is a bare scalar (e.g. 'model: gpt-4o'), running
'hermes config set model.provider openai' silently destroyed the
model id because _set_nested replaced the scalar with an empty dict
before writing the sub-key. Now the scalar is normalized to
{default: <id>} first, preserving the model id.

hermes_cli/config.py

@@ -4860,6 +4860,16 @@ def set_config_value(key: str, value: str, force: bool = False):
             coerced_value = float(value)
 
     value = coerced_value
+    # Normalize a scalar ``model`` key before writing sub-keys so that
+    # ``hermes config set model.provider openai`` doesn't silently
+    # destroy the model id when ``model`` is a bare string shorthand
+    # (e.g. ``model: gpt-4o``).  Without this _set_nested replaces the
+    # scalar with an empty dict, dropping the model id permanently.
+    _model_key = key.strip().lower()
+    if _model_key.startswith("model."):
+        _model_val = user_config.get("model")
+        if isinstance(_model_val, str) and _model_val:
+            user_config["model"] = {"default": _model_val}
     # Guard against #74995: a single-segment key that names an existing
     # mapping would silently overwrite the entire section with a scalar
     # (e.g. ``hermes config set model gpt-5.6-sol`` when model already