feat: add task categorization and tagging

[Chirudeva-Reddy]

Summary

Closes #2.

This PR implements task categorization and tagging across the TaskPilot library, CLI interface, and documentation:

• Task Structure: Supports category and multiple tags in TaskPilot.add_task(task, priority, category=..., tags=...).
• Category Operations: Added set_category(task_number, category), add_tags(task_number, tags), get_categories(), and list_categories().
• Filtering & Display: Added filter_tasks_by_category(category) and updated view_tasks() to cleanly render assigned categories (e.g., [work]) alongside priority and status tags, including [DONE] indicator when showing completed tasks.
• Tools Metadata: Updated src/tools.py (get_available_tools) to document all category and tagging tools for assistant inspection.
• CLI Commands:
  • add <task> [--priority ...] [--category <name>] [--tags <tag1,tag2>]
  • category <task_number> <category>
  • list <category>
  • categories
• Unit Tests: Added comprehensive unit test coverage in tests/test_agent.py under TestCategoryFeatures covering assignment, tags, updating, invalid numbers, filtering, completed task viewing, listing, and tool cataloging.
• Documentation: Updated README.md commands reference with full syntax and examples.

Verification

• uv run pytest: All 21 unit tests pass cleanly.