- Add get_knowledge_directory() utility function following CREWAI_STORAGE_DIR pattern
- Update BaseFileKnowledgeSource, CrewDoclingSource, and ExcelKnowledgeSource to use new function
- Add comprehensive tests for utility function and knowledge source integration
- Maintain backward compatibility with default 'knowledge' directory
- Add proper error handling for non-existent directories
Fixes#3519
Co-Authored-By: João <joao@crewai.com>