feat: add --json output format to all commands

[nguyenquockhang1610]

Summary

Adds --json flag to all CLI commands for scripting and automation (#1).

Usage

# Add task with JSON output
python task.py --json add "Buy groceries"
# {"status": "added", "task": {"id": 1, "description": "Buy groceries", "done": false}}

# List tasks as JSON
python task.py --json list
# {"tasks": [{"id": 1, "description": "Buy groceries", "done": false}]}

# Mark done with JSON output
python task.py --json done 1
# {"status": "done", "task": {"id": 1, "description": "Buy groceries", "done": true}}

Testing

All 6 tests pass including new JSON output tests.

Closes #1