refactor: extract validation and path helpers to utils module

[brutusworker-arch]

Summary

• Extract scattered validation logic and path helpers into a shared utils module
• Eliminates code duplication across command files with no behavior changes

Changes

• Create utils/__init__.py to mark the utils package
• Create utils/paths.py with shared get_tasks_file() helper (previously duplicated in all three command files)
• Create utils/validation.py with validate_description(), validate_task_file(), and validate_task_id() (previously scattered across commands/add.py, commands/list.py, commands/done.py)
• Update commands/add.py to import from utils.paths and utils.validation
• Update commands/list.py to import from utils.validation
• Update commands/done.py to import from utils.paths and utils.validation
• Update test_task.py to import validators from utils.validation

Testing

• python -m pytest test_task.py — all existing tests pass with no modifications to test logic
• Pure refactor: all function signatures, return types, and behaviors are preserved exactly

Closes #3

---

Closes #3

Built by brutusworker-arch