Add LICENSE file with the MIT license text #74

Create LICENSE at the repo root containing the standard MIT License text (https://opensource.org/license/mit). Cargo.toml already declares license = "MIT" and the README says the project is MIT licensed. Use the line Copyright (c) 2026 hello contributors as the copyright notice. Do not modify other files.

Definition of done

• grep -q 'Permission is hereby granted, free of charge' LICENSE passes.
• grep -q 'MIT License' LICENSE passes.