Add workflow to test setup-node@v6 version resolution

[Copilot]

Tests what Node.js version gets installed when using latest or node specifiers with actions/setup-node@v6.

Changes

• Add .github/workflows/test-setup-node.yml with matrix strategy for latest and node versions
• Output installed Node.js and npm versions for comparison
• Explicit contents: read permissions

Workflow

strategy:
  matrix:
    node-version: [latest, node]
steps:
  - uses: actions/setup-node@v6
    with:
      node-version: ${{ matrix.node-version }}
  - run: node --version && npm --version

Triggers on push and manual dispatch.

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