* fix: pin Yarn version from packageManager field in build scripts
Replace hardcoded 'yarn set version 4' in build-functions.sh with a
dynamic lookup from each workspace's package.json packageManager field.
Also syncs runtime yarn version to 4.15.0 to match web/package.json.
Fixes CI build failures caused by Yarn 4.x fetching a newer patch that
produces different builtin compat hashes, breaking --immutable lockfile
validation.