0b0c1b326c
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.