refactor: extract validation and path utilities (fixes #3)

[autogainz]

Summary

Extracted scattered validation logic and duplicate path helpers into dedicated utility modules to improve code organization and reduce duplication.

Changes

• Created commands/utils_validation.py with centralized validation functions:
  • validate_description() - validates task description length and content
  • validate_task_file() - validates tasks file exists
  • validate_task_id() - validates task ID is in valid range
• Created commands/utils_paths.py with path utilities:
  • get_tasks_file() - centralized path to tasks.json (was duplicated in add.py, list.py, done.py)

Benefits

• Eliminates 3 duplicate get_tasks_file() implementations
• Centralizes all validation logic for easier maintenance
• Improves code reusability across commands
• Makes validation rules consistent across the codebase

Testing

Validation functions maintain identical behavior to original scattered implementations. All existing task operations continue to work as expected.

Payment

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