feat: #1 add --json output format for all commands

[tarai-dl]

What does this PR do?

Fixes #1 — Adds --json flag to output valid JSON for scripting and automation.

Changes

• Added --json as a global flag on the main parser
• add --json — outputs single task object: {"id": 1, "description": "...", "done": false}
• list --json — outputs tasks array: [{...}, {...}] (empty array [] if no tasks)
• done --json — outputs updated task object with done: true
• All existing text output behavior preserved when --json is not used

Usage

python3 task.py --json add "buy groceries"
# {"id": 1, "description": "buy groceries", "done": false}

python3 task.py --json list
# [{"id": 1, "description": "buy groceries", "done": false}]

python3 task.py --json done 1
# {"id": 1, "description": "buy groceries", "done": true}

Tests

• 7 tests all pass
• New tests: test_add_json_output, test_list_json_output, test_list_json_empty, test_done_json_output, test_cli_json_flag

Fixes #1