feat: add --json output format

[realkoreanbeef]

Summary

• Adds a --json flag for add, list, and done commands
• Emits a consistent {success, message, data} JSON envelope for scripting
• Preserves existing human-readable output by default
• Documents JSON usage examples in the README

Verification

• Static scan for hardcoded secrets / shell injection / eval / pickle / SQL injection: OK
• python3 -m pytest -q -> 11 passed
• python3 -m py_compile task.py commands/add.py commands/done.py commands/list.py commands/output.py commands/__init__.py test_task.py
• Manual JSON parse smoke test for add, list, and done
• Independent code review passed with no blocking security concerns or logic errors

Closes #1