Sandboxed code migration agent cookbook — artifact validation for migration outputs

AI Impact Summary

The new sandboxed code migration agent cookbook adds automated artifact validation for migration tasks. The validation enforces that outputs include migration_report.md, migration.patch, migration_result.json, and migration_audit.jsonl, and that patches contain specific module markers for support_reply_service and case_summary_service, as well as test references under tests/ and unittest/compileall checks. It also requires a host_artifacts_written event in the audit log, ensuring an end-to-end, auditable migration workflow. This strengthens the migration workflow for the customer_support_bot and case_summary_service components by preventing incomplete or non-compliant patches from advancing.