Add helper that picks primary model with fallback to a secondary model
(e.g., categoryModel → parentContext.model). Enforces consistent
{providerID, modelID} shape.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>