ci: bump checkout/setup-node to v6 (Node 24 runtime)

GitHub deprecated Node.js 20 for actions: actions/checkout@v4 and
actions/setup-node@v4 run on Node 20 and emit a deprecation warning.
Node 24 becomes the forced default on 2026-06-02 and Node 20 is removed
on 2026-09-16. Bump both to @v6 (Node 24). Config is unchanged —
node-version: 22 and registry-url are both supported in v6.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>