Sprint 5C: Task Artifact Records and Registration

[samrusani]

Summary

• add task_artifacts migration, contracts, store support, and rooted registration service
• add artifact register/list/detail API routes and deterministic duplicate handling
• add unit and integration coverage plus build/review reports and architecture updates

Verification

• ./.venv/bin/python -m pytest tests/unit
• ./.venv/bin/python -m pytest tests/integration
• git diff --check