Feature: Add Due Date Support for Tasks

[Sapna700]

Description

Add support for due dates on tasks so users can set deadlines and get reminders about upcoming tasks.

Goals

• Add due date field to tasks
• Display due dates in task list
• Show overdue tasks with warning
• Sort tasks by due date
• Optional: Add reminders/notifications

Implementation Details

• Use Python's datetime module
• Update task structure to include due_date
• Add CLI commands:
  • add <task> --due YYYY-MM-DD
  • due <task_number> <date>
  • overdue - Show overdue tasks
  • upcoming - Show upcoming tasks

Acceptance Criteria

• ✅ Tasks can have due dates
• ✅ Due dates display in task list
• ✅ Overdue tasks are highlighted
• ✅ Tasks can be sorted by due date
• ✅ Date validation works correctly
• ✅ Unit tests added
• ✅ Documentation updated

Example Usage

You: add Complete project --due 2026-09-15
Task added: Complete project (Due: 2026-09-15)

You: tasks
1. Complete project (Due: 2026-09-15) - 15 days left
2. Buy groceries (OVERDUE - 2 days)

Related Issues

Part of TaskPilot-AI roadmap