Add JSON output format

[xiaopeng215-sys]

Closes #1.

Summary

• Added --json support for add, list, and done while preserving existing human-readable output by default.
• JSON responses include task payloads for scripting and automation.
• Added CLI regression tests for valid JSON output and documented examples in the README.

Verification

• python3 -m py_compile task.py commands/add.py commands/list.py commands/done.py
• uvx --from pytest pytest test_task.py -q
• Manual JSON smoke test for add --json, list --json, and done --json with json.loads
• git diff --check