docs: archive completed roadmap audit

[jerdaw]

Summary

• archive the completed February 2026 roadmap audit under the planning archive naming convention
• add required archived-document metadata
• index the historical record from the planning and archive READMEs
• leave current roadmap priorities, service data, runtime behavior, and operations unchanged

Validation

• npm run lint
• npm run type-check
• npm run format:check
• npm run check:refs
• npm run check:root
• npm test -- --run tests/unit/documentation-hygiene.test.ts
• npm run build
• full pre-push Vitest suite: 219 files, 1,746 passed, 24 credential-gated skipped