Refactor validation into shared utils

[sureshchouksey8]

Summary

• Add a utils package with shared paths and validation modules.
• Move task description, task file, and task ID validation out of command modules.
• Update add/list/done commands to import shared helpers without changing behavior.
• Update tests to import validators from the shared module and cover missing task-file behavior.

Verification

• python3 -m pytest -q test_task.py - 3 passed
• python3 -m py_compile task.py commands/*.py utils/*.py test_task.py
• git diff --check

/claim #3