fix(cli): prevent crash in save_config_value when model is a string

load_cli_config() supports both string and dict formats for the model
key (e.g. `model: "anthropic/claude-opus-4"`), but save_config_value()
assumed all intermediate keys are dicts. When the config file used the
string format, running `/model <name>` would crash with TypeError:
'str' object does not support item assignment.

Add an isinstance check so non-dict values are replaced with a fresh
dict before descending.

cli.py

@@ -708,7 +708,7 @@ def save_config_value(key_path: str, value: any) -> bool:
         keys = key_path.split('.')
         current = config
         for key in keys[:-1]:
-            if key not in current:
+            if key not in current or not isinstance(current[key], dict):
                 current[key] = {}
             current = current[key]
         current[keys[-1]] = value