fix: re-implement loading Roo modes from .roo/modes directories

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

This re-implements the feature that was reverted in #7332, addressing the issue
reported by @farazoman where YAML files in $HOME/.roo/modes were not loading.

Fixes #7202

Author: roomote