feat: add --json output flag to all commands (Closes #1)

[alexanderxfgl-bit]

What

Adds --json flag to all CLI commands (add, list, done) for structured output.

Changes

• task.py: Add global --json argument, pass json_output to commands
• commands/add.py: Return task dict as JSON when flag set
• commands/list.py: Return tasks array as JSON when flag set
• commands/done.py: Return task dict or error object as JSON when flag set
• test_task.py: 6 new tests covering JSON output for all commands + edge cases

Testing

8 passed in 0.03s

Example

$ python task.py add "Build API" --json
{"id": 1, "description": "Build API", "done": false}

$ python task.py list --json
[{"id": 1, "description": "Build API", "done": false}]

$ python task.py done 1 --json
{"id": 1, "description": "Build API", "done": true}

Closes #1