feat: enable loading Roo modes from multiple files in .roo/roo_modes directory (#7203)

* 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

* refactor: rename .roo/roo_modes to .roo/modes for cleaner directory structure

- Changed ROO_MODES_DIR constant from "roo_modes" to "modes"
- Updated all references in CustomModesManager.ts
- Updated all test descriptions and mocks in CustomModesManager.spec.ts
- All tests passing successfully

* fix: normalize sourceFile paths to use forward slashes in exported YAML

- Fixes Windows test failure where backslashes in sourceFile paths were causing test assertions to fail
- Ensures cross-platform compatibility by normalizing all paths to use forward slashes in YAML exports

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Co-authored-by: Roo Code <[email protected]>
Co-authored-by: daniel-lxs <[email protected]>

Author: 3 people