Path to the user's model configuration file (~/.deepagents/config.toml).
Parsing for the [sandboxes] section of ~/.deepagents/config.toml.
Parallels the [models] provider configuration in model_config.py. Config
providers declare a class_path (same trust model as model class_path),
a working_dir, an optional install package, and params forwarded to
provider.get_or_create().
