Replace complex fallback logic with new shared model resolution utilities. The model-fallback module now delegates to resolveModelWithFallback() for cleaner, consistent model selection across the codebase.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
