Add sc-refactory package and runtime support

[randlee]

Summary

• add the sc-refactory package with commands, skills, agents, scripts, assets, and design docs
• add installer/runtime support for plugin artifacts and complete the refactory repo installer flow
• merge develop, update registry/marketplace metadata, and align validators/package discovery for the new package

Validation

• python3 scripts/validate-all.py --parallel --continue-on-failure
• pytest -q tests/test_sc_install.py tests/test_sc_install_phase1_2.py
• python3 packages/sc-refactory/tests/test_package_layout.py
• python3 packages/sc-refactory/scripts/validate_agents.py

Notes

• sync_subset.py is implemented as a first practical local subset-sync path
• named teammate/orchestration content is packaged and validated, but still early compared with older packages