Handle missing config file gracefully

[nguyenduchuyhd]

Closes #2

Summary

• Creates a sensible default config when ~/.config/task-cli/config.yaml is missing.
• Loads the generated config instead of crashing with FileNotFoundError.
• Adds regression coverage and documents the default behavior.

Verification

• python -m pytest test_task.py
• Manual smoke with a temporary home directory: python task.py list