chore: add a LICENSE file to the repository

[RyanBayne]

Summary

The repository has no LICENSE file. This is required before going public so contributors and users understand the terms of use.

What to do

1. Decide on a license (MIT, Apache 2.0, or AGPL-3.0 are common choices for open-source tools)
2. Add a LICENSE file to the root of the repository
3. Optionally reference the license in README.md

Acceptance Criteria

• A LICENSE file exists at the repository root
• README.md mentions the license