Add JSON output mode

[thunderisme]

Closes #1
/claim #1

Summary

• Adds --json support to add, list, and done.
• Returns parseable JSON envelopes while preserving existing text output by default.
• Adds CLI-level regression tests for JSON add/list/done output.
• Updates README with JSON usage examples.

Validation

• HOME=$(mktemp -d) uvx --from pytest pytest -q test_task.py
• Manual JSON checks for add, list, and done with isolated temporary HOME
• git diff --check