feat: add JSON output mode

[nicovaleops]

Closes #1.

This PR adds machine-readable JSON output across all three task commands.

Changes:

• adds --json to add, list, and done
• emits parseable JSON for created/listed/completed tasks
• preserves existing human-readable output by default
• documents JSON examples in the README
• adds CLI-level tests for add/list/done JSON flows and empty list state

Validation:

• python3 -m compileall task.py commands test_task.py test_json_output.py
• python3 -m pytest -q → 4 passed