chore: remove obsolete unmonorepo tooling

[jderochervlk]

Tracking issue: #342

Summary

• remove the one-shot scripts/unmonorepo migration utilities now that the folder and feature migration is complete
• remove the dedicated migration tests that only exercised those obsolete utilities
• leave production source, package configuration, and the historical design record unchanged

Stack context

• final cleanup layer in stack #344
• depends on the completed folder and feature migration below it

Verification

The current head passes the full CI build, including:

• npm run format:check
• npm run build
• npm run check:features
• npm test
• npm run build:docs
• npm pack