Add JSON output format

[nguyenduchuyhd]

Closes #1

Summary

• Adds a --json flag to add, list, and done commands.
• Emits parseable JSON for task creation, listing, completion, and empty/error states.
• Updates README examples for automation usage.

Verification

• python -m pytest test_task.py
• Manual smoke: python task.py add "Alpha task" --json; python task.py list --json; python task.py done 1 --json