Add JSON output mode

[letrquan]

Summary

• Add --json support to add, list, and done commands.
• Emit parseable JSON payloads for created, listed, and completed tasks.
• Add tests and README examples for JSON output mode.

Validation

• python -m py_compile task.py test_task.py commands/add.py commands/done.py commands/list.py
• HOME="$PWD/.tmp-home" python task.py add "Buy groceries" --json
• HOME="$PWD/.tmp-home" python task.py list --json
• HOME="$PWD/.tmp-home" python task.py done 1 --json

/claim #1