Add download smoke execute validation handoff

[1613358894]

Summary

• add structured validation-only download-smoke execute handoff fields to prepare --write output
• keep the recommended execute handoff free of --execute so it validates the pinned command manifest without downloads
• document the handoff and update focused download-smoke tests

Validation

• python -m pytest tests/test_cli_download_smoke.py -q --basetemp .pytest_tmp_download_smoke_execute_handoff -p no:cacheprovider
• python -m pytest tests/test_docs_consistency.py tests/test_docs_hygiene_script.py -q --basetemp .pytest_tmp_download_smoke_execute_handoff_docs -p no:cacheprovider
• python scripts/check_docs_hygiene.py
• python scripts/check_workspace_hygiene.py
• git diff --check
• no-download Clostridium prepare handoff rerun against E:\scratch\clostridium_download\cache\ncbi\download_plan.tsv