fix(agent): separate preset snapshots from config reload

Co-authored-by: Cursor <cursoragent@cursor.com>

nanobot/config/schema.py

@@ -281,6 +281,8 @@ class Config(BaseSettings):
 
     @model_validator(mode="after")
     def _validate_model_preset(self) -> "Config":
+        if "default" in self.model_presets:
+            raise ValueError("model_preset name 'default' is reserved for agents.defaults")
         name = self.agents.defaults.model_preset
         if name and name != "default" and name not in self.model_presets:
             raise ValueError(f"model_preset {name!r} not found in model_presets")