Railway rootDirectory isolates build context — postinstall cp from
../shared/ fails because parent dirs aren't in the Nixpacks image.
Commit JSON/CJS configs directly into scripts/shared/.
- Remove useless postinstall from scripts/package.json
- Remove scripts/shared/ from .gitignore
- Commit all shared config files into scripts/shared/
- Add sync test to catch drift between shared/ and scripts/shared/
