tailwind-v3-to-v4-migration

# tailwind-v3-to-v4-migration Upgrade a codebase from Tailwind CSS v3 to v4. The codemod does ~80% of the mechanical work; this skill supplies the 20% of judgment where migrations actually break — changed defaults, config porting, plugin/animation swaps, and proving nothing moved. ## When to use - Upgrading any project from Tailwind v3.x to v4.x. - Build errors after a partial upgrade: `@tailwind` directives unknown, missing `@tailwindcss/postcss`, `Cannot apply unknown utility class`, `tailwind.config` no longer picked up. - Converting `tailwind.config.{js,ts}` to CSS-first `@theme`. Skip if: the project is already on v4; you need to *downgrade*; or you are building a brand-new design system rather than migrating one. Note v4 targets **Safari 16.4+, Chrome 111+, Firefox 128+** — if you must support older browsers, stay on v3.4 (flag this to the operator before proceeding). ## The one idea that makes this safe **A correct migration is a visual no-op.** Every renamed utility is a pure alias — `shadow-sm`→ `shadow-xs`, `rounded`→`rounded-sm`, `ring`→`ring-3`, `outline-none`→`outline-hidden` all compile to the *same* CSS as before. So what changes pixels is almost entirely v4's **changed defaults** (Step 3); the few non-default exceptions — the `space-x/y-*` & `divide-*` selector change, gradient-variant preservation, and `container` config removal — are flagged in Step 4. Rename mechanically, neutralize the changed defaults, fix those few exceptions, and the rendered ou...