Refactor validation and path helpers

[darshan-Jahagirdar]

Fixes #3.

Changes

• Added utils/validation.py for shared validation helpers.
• Added utils/paths.py for the shared task storage path helper.
• Updated add/list/done commands to import the shared helpers.
• Updated tests to import validators from the new shared module.

Verification

• python -m pytest test_task.py -> 2 passed
• rg "def validate_|def get_tasks_file|Validation logic scattered" commands utils test_task.py shows helper definitions only under utils/

/claim #3

[darshan-Jahagirdar]

Validation update for this PR:

ext py -3.11 -B -m pytest test_task.py git diff --check

Result: 2 tests passed and the diff whitespace check passed. I installed pytest into a temporary test-only directory for this run and removed it afterward.