docs: remove empty economics book placeholder

[unityequilibrium]

Summary

• remove the empty public origin_of_wealth_and_economics placeholder
• keep the public books tree limited to reviewed scaffolds
• record the economics/wealth-origin draft as excluded local material that still needs encoding, claim, and source-boundary review

Notes

This does not delete the local draft material. It only prevents GitHub main from showing an empty misleading book folder.

Test Plan

• checked branch diff scope against origin/main
• checked uet_history/3_publish/books tree only contains