docs: update monorepo documentation to reflect Tailwind v4 migration

## Summary

Several documentation files still reference the old Tailwind v3 setup after the v4 migration (#842). These should be updated so consumers and contributors get accurate information.

## Files to Update

### Root README.md
- [ ] Update dependency graph (mermaid) to remove `design_system_react --> design_system_tailwind_preset` edge
- [ ] Mark `@metamask/design-system-tailwind-preset` as deprecated/v3-only in the package list (or defer to #981)

### CLAUDE.md
- [ ] Update package descriptions to note that `design-system-tailwind-preset` is v3-only/deprecated
- [ ] Note that `design-tokens` now exports `tailwind/theme.css` for v4 consumers

### design-tokens/README.md
- [ ] Add a "Tailwind CSS v4" section showing how to use `@metamask/design-tokens/tailwind/theme.css`
- [ ] Update the existing "CSS Variables" section to note that Tailwind v4 consumers should use `theme.css` instead of `styles.css`

### design-system-react/README.md
- [ ] Add setup instructions noting Tailwind v4 and `@metamask/design-tokens` as requirements
- [ ] Link to the migration guide for consumers upgrading from v3

## Context

The migration guides (`design-tokens/MIGRATION.md` and `design-system-react/MIGRATION.md`) are already comprehensive. This issue covers the README and contributor docs that weren't updated as part of #842.

Start with the four named files: README.md, CLAUDE.md, design-tokens/README.md, and design-system-react/README.md. Compare their Tailwind v3 references with the existing design-tokens/MIGRATION.md and design-system-react/MIGRATION.md guidance. Done means the dependency graph, package descriptions, v4 theme.css usage, setup requirements, and migration link are accurate across the checklists.

Written by the indexing model from the issue text.