Lifts ptpa_pl's local .gitea/actions/pnpm-install into this repo, parametrised
enough for repositories we have not met yet: working-directory, install args,
pnpm version and an opt-in store cache.
Cache is off by default. ADR-0014 pins actions/cache to major 3 and no workflow
on this instance has exercised the cache server yet - a shared action that a
dozen jobs hang off is the wrong place to premiere that dependency.
Two traps found while testing against ptpa_pl's lockfile in node:24-bookworm-slim,
both avoided by pinning the store instead of reading `pnpm store path`:
that command defaults to <workspace>/.pnpm-store, inside the working tree where a
committing job could sweep it up, and it exits 0 while printing its error to
stdout, which would have fed actions/cache a garbage path silently.
The version input also carries COREPACK_ENABLE_PROJECT_SPEC=0, without which a
packageManager field outranks `corepack prepare --activate` and the input would
do nothing at all - no effect, no error.
cierzniak