feat: add configurable UI design policy

[whitelonng]

Summary

• add an opt-in, revisioned UI design policy with preserve, refine, and experimental presets
• require 2–3 visual direction proposals before new UI or aesthetic redesign work when no direction is selected
• strengthen experimental guidance for memorable first viewports and cohesive full-page motifs while preserving workflow clarity
• add scoped monorepo style scans and mixed icon-system diagnostics
• propagate bounded design context across supported agent adapters

Validation

• npm run lint
• npm run typecheck
• npm run build
• npm run test:dist
• npm run test:windows-smoke
• npm test -- --silent (122 files, 857 tests)

Review

• completed a final diff review; no unresolved P0/P1 findings
• design policy remains opt-in and fails open to the safe preserve preset