Corepack is gone from Node 25+, so every `corepack enable` call
fails. pnpm now ships as a system binary (devenv, CI runners and
Docker images install it directly) and auto-downloads the version
pinned in `packageManager` on mismatch.
Scripts, workflows and Dockerfiles call `pnpm` straight away; the
three deploy workflows use a single `pnpm/setup@v2` step; and the
new `scripts/sync-pnpm-version` stamps all 35 `packageManager`
fields from the system pnpm, replacing the `corepack use` sweep.
AI-assisted-by: muse-spark-1.3-contributor