Yarn Berry removed the --ignore-engines flag; engine checking is no
longer a core feature. The deprecated flag causes yarn install to exit
with error code 1.
All 18 pnpm/yarn CI jobs fail on main because:
1. pnpm v9+ refuses to install when package.json declares
"packageManager": "yarn@4.9.2" — fixed by setting
COREPACK_ENABLE_STRICT=0 and --no-frozen-lockfile
2. CI runners only have Yarn Classic (v1.x) but the project
uses Yarn Berry (v4.x) — fixed by activating Corepack
before the cache/install steps
