feat: add --json output, fix config crash, extract validation utils

[flaggdavid-source]

Summary

Fixes #1, Fixes #2, Fixes #3

Three bounty issues resolved in one cohesive PR since the refactor (#3) is a dependency for clean implementations of the other two.

Changes

Issue #1 ($50) — Add --json output format

• --json flag added to all subcommands (add, list, done)
• list --json outputs valid JSON array of tasks
• add --json and done --json output status confirmation objects
• Human-readable output unchanged when flag is absent

Issue #2 ($75) — Fix crash when config file missing

• load_config() now creates default config if ~/.config/task-cli/config.yaml is missing
• Default values match config.yaml.example
• Parent directories created automatically via mkdir(parents=True)

Issue #3 ($100) — Extract validation to utils module

• New commands/utils.py with shared functions: validate_description(), validate_task_id(), get_tasks_file(), load_tasks(), save_tasks()
• Removed duplicate get_tasks_file() from add.py, list.py, done.py
• Removed scattered validation from individual command files
• All commands now import from commands.utils
• Added .gitignore for __pycache__/

Testing

8 tests, all passing:

test_validate_description        PASSED
test_validate_task_id            PASSED
test_load_config_creates_default PASSED
test_load_config_reads_existing  PASSED
test_list_tasks_json             PASSED
test_list_tasks_json_empty       PASSED
test_list_tasks_human            PASSED
test_save_and_load_tasks         PASSED

Demo

$ python task.py add "Buy groceries"
Added task 1: Buy groceries

$ python task.py list --json
[
  {"id": 1, "description": "Buy groceries", "done": false}
]

$ python task.py done 1 --json
Marked task 1 as done: Buy groceries
{"status": "done", "task_id": 1}