Design a task management system with tasks broken into sub-tasks.

A robust task management system would incorporate a hierarchical structure, allowing tasks to be decomposed into smaller, manageable sub-tasks. This design supports clear delegation, progress tracking at granular levels, and improved project organization. Key features would include task creation, assignment, due dates, status updates, and the ability to link sub-tasks to their parent tasks. Notifications and reporting functionalities would further enhance user experience and project oversight.