[quality] Add integration test job to nightly CI workflow

[clubanderson]

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---## Finding

The internal/runner package has a 201-line integration test file guarded by //go:build integration. These tests exercise real process-group cancellation, context deadlines, and mid-write cancellations.

No CI workflow runs them. The nightly workflow only runs go test -race which excludes integration-tagged tests.

Recommendation

Add a nightly-integration job to .github/workflows/nightly.yml:

  nightly-integration:
    name: nightly integration tests
    runs-on: ubuntu-latest
    timeout-minutes: 10
    steps:
      - uses: actions/checkout@v6
      - uses: actions/setup-go@v6
        with:
          go-version: "1.26"
      - name: integration tests
        run: go test -tags=integration -race -count=1 ./...

Priority

• Impact: high
• Effort: low

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Filed by quality agent (hold-gated mode)