Extract shared validation utilities

[Wheezily]

Closes #3.

Changes:

• Adds commands/utils.py for shared validation helpers.
• Reuses validate_description, validate_task_id, and get_tasks_file from command modules.
• Updates tests to import validation helpers from the shared module.

Validation:

• python -m compileall task.py commands test_task.py
• direct smoke test for the validation helpers

Note: pytest is not installed in this local environment, so I validated with standard Python checks.