chore: remove obsolete unmonorepo tooling (#345)

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`

Author: jderochervlk