Expose download smoke execute handoff contract

[1613358894]

Summary

• include the prepare-stage execution validation handoff fields in commands render output contracts
• add focused coverage so AI metadata stays aligned with download-smoke prepare stdout

Validation

• python -m pytest tests/test_cli_commands.py -k download_smoke -q --basetemp .pytest_tmp_commands_download_smoke_contract -p no:cacheprovider
• python -m pytest tests/test_cli_download_smoke.py -q --basetemp .pytest_tmp_download_smoke_contract -p no:cacheprovider
• python scripts/check_workspace_hygiene.py
• python scripts/check_docs_hygiene.py
• git diff --check
• commands render check using real Clostridium strategy recommended_request