- Add abstract readlines method to Fs class
- Implement readlines in PathFs using file.readlines()
- Implement readlines in RecordFs using splitlines(keepends=True)
- Add comprehensive tests for both implementations
- Test various scenarios: empty files, no final newline, different line endings

* Abstract filesystem operations in file_utils.py

- Add fs parameter to walk_with_gitignore and get_files functions
- Replace direct filesystem calls with Fs interface methods
- Use fs.stat(), fs.readlines(), and fs.list() instead of os.scandir() and open()
- Exclude .gitignore files from walk results
- Add comprehensive tests using RecordFs for deterministic testing
- Test gitignore filtering, skip functions, and edge cases

* Address PR feedback: use fully qualified imports, absolute paths, and add nested gitignore test

- Use fully qualified imports in file_utils.py and file_utils_test.py
- Convert all test paths to absolute paths as requested
- Add comprehensive nested gitignore test case
- Improve RecordFs.list() method to handle intermediate directories properly