Add JSON output format

[sureshchouksey8]

Summary

• Add --json support for add, list, and done commands while preserving existing human-readable output by default.
• Emit parseable JSON payloads for successful add/list/done flows.
• Add tests covering JSON output for all three commands and README examples.

Verification

• python3 -m pytest -q test_task.py - 5 passed
• python3 -m py_compile task.py commands/*.py test_task.py
• git diff --check

/claim #1