Adds a packageManager field (pnpm@9.15.9) to package.json so corepack
and pnpm/action-setup@v4 resolve a single pinned pnpm 9 version that
matches lockfileVersion 9.0. Removes the floating 'version: latest' from
the CI and screenshots workflows so they honor the packageManager field
instead of pulling pnpm 10+, which breaks native module builds.
Follow-up to #63.