Refactor validation helpers into utils

[phungkaizen]

Summary

• Add a utils package for shared task CLI helpers.
• Move the task file path helper into utils/paths.py.
• Move validate_description, validate_task_file, and validate_task_id into utils/validation.py.
• Update command modules and tests to import the shared helpers without changing behavior.

Fixes #3
/claim #3

Review & Testing Checklist for Human

• Confirm add, list, and done still behave the same from the CLI.
• Check there is no remaining duplicate validation/path helper logic in command files.
• Run the existing pytest suite after merge.

Notes

Tested locally:

• python3 -m pytest test_task.py
• python3 -m py_compile task.py commands/add.py commands/list.py commands/done.py utils/paths.py utils/validation.py
• python3 task.py --help
• python3 task.py list
• git diff --check