Artifact retention: set explicit retention-days to control storage costs

GitHub Actions artifacts default to 90-day retention. Large build outputs (binaries, coverage reports, test results) accumulate silently and consume storage quota, eventually triggering billing surprises or hard limits.

Always set retention-days explicitly based on artifact purpose:

- uses: actions/upload-artifact@v4
  with:
    name: build-output-${{ github.run_id }}
    path: dist/
    retention-days: 7   # short-lived build artifacts

- uses: actions/upload-artifact@v4
  with:
    name: release-${{ github.ref_name }}
    path: release/*.tar.gz
    retention-days: 90  # release assets kept longer

For download in the same workflow:

- uses: actions/download-artifact@v4
  with:
    name: build-output-${{ github.run_id }}
    path: dist/

Short-lived CI artifacts (coverage, logs, bundles) don't need 90 days. Setting 1-7 days for transient artifacts dramatically reduces storage consumption.