Polish repository README and remove unused scaffolding

[130U]

Summary

• replace the developer-facing root README with a concise public introduction and the existing optimized profile banner
• remove the empty types/ placeholder and the unused published-copy snapshot scaffold
• rename the active preservation guard to check-protected-sources and align CI, tests, and maintenance documentation
• keep Education, Past Experience, Current Chapter, website code, public assets, rollback tags, and deployment behavior unchanged

Repository audit

The tracked tree contains no duplicate, backup, draft, or generated files beyond the two removed placeholders. All remaining top-level folders participate in source, build, validation, deployment, public assets, or maintenance. Three historical feature branches will be deleted after this PR is merged; annotated rollback tags will remain.

Validation

• npm run check — static export plus 16/16 artifact tests passed
• npm audit --omit=dev --audit-level=moderate — 0 vulnerabilities
• npm run check:protected-sources — protected Education, Past Experience, and Current Chapter sources unchanged
• git diff --check — passed