refactor: extract shared validation utilities

[autochamchikim-pixel]

Summary

• Adds utils/validation.py for shared validation helpers.
• Adds utils/paths.py for the shared task file path helper.
• Updates commands to import the shared helpers instead of duplicating validation/path code.
• Updates tests to import validators from the new shared module.

Test Plan

• uvx --from pytest pytest -q test_task.py
• Manual smoke test: python3 task.py list, python3 task.py add 'Refactor utils check', python3 task.py list, python3 task.py done 1

/claim #3