fix: Refactor validation to utils module + fix config crash (#2, #3)

[zlplzp123wyt]

Summary

This PR fixes two bounty issues:

Issue #3 ($100) - Refactor: Extract validation to utils module

• Created utils/validation.py with validate_description, validate_task_id, validate_task_file
• Created utils/paths.py with get_tasks_file, get_config_path, load_config
• Removed duplicate get_tasks_file() from all three command files (add.py, list.py, done.py)
• Updated all command imports to use shared utilities
• No behavior changes - pure refactor
• All existing tests pass + added new tests

Issue #2 ($75) - Fix: Crash when config file missing

• load_config() now returns None when config doesn't exist instead of crashing with FileNotFoundError
• task.py shows a helpful warning message when config is missing:

  Warning: No config file found at ~/.config/task-cli/config.yaml
  Using default settings. Create a config file to customize behavior.
  

Changes

• New files: utils/__init__.py, utils/validation.py, utils/paths.py, .gitignore
• Modified: task.py, commands/add.py, commands/list.py, commands/done.py, test_task.py
• Tests: All 5 tests pass (2 original + 3 new for utils)

$ python3 -m pytest test_task.py -v
test_task.py::test_validate_description PASSED
test_task.py::test_validate_task_id PASSED
test_task.py::test_get_tasks_file PASSED
test_task.py::test_get_config_path PASSED
test_task.py::test_load_config_missing PASSED

/claim #2 #3