feat: add Cursor worker engine

[victurbo37-debug]

Summary

• add a built-in Cursor worker engine using stream JSON output
• map Ringer sandbox modes to Cursor's sandbox options, including the existing explicit full-access gate
• document configuration and aliases and add focused command-construction tests

Context

This is split 2 of 5 from #31, following the maintainer's request for independently reviewable changes.

Validation

• python -m unittest tests.test_cursor_engine -v — 3 passed
• parsed the updated config.sample.toml