feat: add --json output flag to all commands

[rainlee]

Closes #1

Changes

Added --json flag support to add, list, and done commands.

Usage:

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

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

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

Tests

Added 3 new tests covering JSON output for add, list, and empty list cases.
All 7 tests pass.