Replace empty AppConfig stub with ColorConfig struct that stores level-to-color mappings as strings. Supports loading/saving from XDG config directory via TOML, with graceful degradation on missing/invalid files. Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent) Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>