Fix reusable Node CI pnpm cache detection

[ArchonVII]

Problem

repo-required-gate delegates Node checks to the reusable node-ci.yml workflow, but node-ci currently defaults actions/setup-node to cache: npm before it detects the repository package manager. pnpm-only repos with pnpm-lock.yaml and no npm lockfile fail before install starts.

Acceptance Criteria

• Reusable Node CI auto-selects the setup-node cache package manager from the lockfile before calling setup-node.
• pnpm-only repos no longer fail setup-node because an npm lockfile is absent.
• The repo-required-gate Node lane inherits the fixed behavior without every consumer adding bespoke inputs.
• Tests and/or YAML validation cover the workflow change.

Evidence

Observed from ArchonVII/jma-ui PR #27: repo-required-gate / node ci fails because setup-node is using cache: npm against a pnpm-only repo.