Support organizing commands in subdirectories with colon-separated naming
(e.g., myproject/deploy.md becomes myproject:deploy).
- Recursively traverse subdirectories and load all .md command files
- Prefix nested command names with directory path (colon-separated)
- Protect against circular symlinks via visited path tracking
- Skip hidden directories (consistent with other loaders)
- Graceful error handling with logging for debugging
