Add CI job to run tests on minimum supported Node.js version (v20)

[Copilot]

CI only ran tests against the runner's default Node.js version, with no coverage for the minimum supported version.

Changes

• .github/workflows/ci.yml: Added tests_min_node job ("Tests (Node 20)") — a duplicate of the existing tests job, pinned to Node.js 20 via wyvox/action-setup-pnpm@v3's node-version input. Runs in parallel with tests, both gated on setup.

tests_min_node:
  name: "Tests (Node 20)"
  runs-on: ubuntu-latest
  needs: [setup]
  steps:
    - uses: actions/checkout@v4
    - uses: wyvox/action-setup-pnpm@v3
      with:
        node-version: 20
    - run: pnpm test
    - run: pnpm --filter '*' test

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