docs: move attribution notice out of license

[2023Anita]

Summary

• keep LICENSE as the standard MIT license text only
• move the Superpowers architecture attribution into a separate NOTICE file
• link both LICENSE and NOTICE from the README license section

Motivation

The README and package manifests identify the project as MIT licensed, but GitHub currently reports the repository license as Other. Keeping the license file as the standard MIT template should make license detection clearer while preserving the attribution note in a conventional place.

Verification

bash scripts/bump-version.sh --check
git diff --check

I also checked locally that:

• LICENSE no longer contains the attribution note
• NOTICE exists
• README.md links to NOTICE