Extract shared validation utilities

[letrquan]

Summary

• Move shared task file path resolution into utils.paths.
• Move validation helpers into utils.validation.
• Update commands and tests to use the shared utilities without behavior changes.

Validation

• python -m py_compile task.py test_task.py commands/add.py commands/done.py commands/list.py utils/init.py utils/paths.py utils/validation.py
• HOME="$PWD/.tmp-home" python task.py add "Refactor validation"
• HOME="$PWD/.tmp-home" python task.py list
• HOME="$PWD/.tmp-home" python task.py done 1

/claim #3