feat: enable loading Roo modes from multiple files in .roo/roo_modes directory

- Add support for loading modes from .roo/roo_modes directory (both global and project)
- Implement proper precedence: project .roo/roo_modes > .roomodes > global .roo/roo_modes > settings
- Update file watchers to monitor .roo/roo_modes directories
- Preserve original source file when updating modes
- Add comprehensive tests for the new functionality

Fixes #7202

Author: roomote