Handle missing config file gracefully

[letrquan]

Summary

• Create a default ~/.config/task-cli/config.yaml when the config file is missing.
• Add regression coverage for missing config creation.
• Update configuration docs to describe the default file behavior.

Validation

• python -m py_compile task.py test_task.py commands/add.py commands/done.py commands/list.py

Note: python -m pytest test_task.py could not be run locally because pytest is not installed in this environment.

/claim #2