fix: openclaw migration overwrites model config dict with string (#3924)

migrate_model_config() was writing `config["model"] = model_str` which
replaces the entire model dict (default, provider, base_url) with a
bare string. This causes 'str' object has no attribute 'get' errors
throughout Hermes when any code does model_cfg.get("default").

Now preserves the existing model dict and only updates the "default"
key, keeping provider/base_url intact.

optional-skills/migration/openclaw-migration/scripts/openclaw_to_hermes.py

@@ -1297,7 +1297,11 @@ class Migrator:
 
         if self.execute:
             backup_path = self.maybe_backup(destination)
-            hermes_config["model"] = model_str
+            existing_model = hermes_config.get("model")
+            if isinstance(existing_model, dict):
+                existing_model["default"] = model_str
+            else:
+                hermes_config["model"] = {"default": model_str}
             dump_yaml_file(destination, hermes_config)
             self.record("model-config", source_path, destination, "migrated", backup=str(backup_path) if backup_path else "", model=model_str)
         else: