Add PostCSS configuration and skills lock file

- Created a new PostCSS configuration file to integrate Tailwind CSS.
- Added a skills lock file containing various Expo skills with their respective source and computed hashes.

.agents/skills/upgrading-expo/references/react-19.md

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+# React 19
+
+React 19 is included in Expo SDK 54. This release simplifies several common patterns.
+
+## Context Changes
+
+### useContext → use
+
+The `use` hook replaces `useContext`:
+
+```tsx
+// Before (React 18)
+import { useContext } from "react";
+const value = useContext(MyContext);
+
+// After (React 19)
+import { use } from "react";
+const value = use(MyContext);
+```
+
+- The `use` hook can also read promises, enabling Suspense-based data fetching.
+- `use` can be called conditionally, this simplifies components that consume multiple contexts.
+
+### Context.Provider → Context
+
+Context providers no longer need the `.Provider` suffix:
+
+```tsx
+// Before (React 18)
+<ThemeContext.Provider value={theme}>
+  {children}
+</ThemeContext.Provider>
+
+// After (React 19)
+<ThemeContext value={theme}>
+  {children}
+</ThemeContext>
+```
+
+## ref as a Prop
+
+### Removing forwardRef
+
+Components can now receive `ref` as a regular prop. `forwardRef` is no longer needed:
+
+```tsx
+// Before (React 18)
+import { forwardRef } from "react";
+
+const Input = forwardRef<TextInput, Props>((props, ref) => {
+  return <TextInput ref={ref} {...props} />;
+});
+
+// After (React 19)
+function Input({ ref, ...props }: Props & { ref?: React.Ref<TextInput> }) {
+  return <TextInput ref={ref} {...props} />;
+}
+```
+
+### Migration Steps
+
+1. Remove `forwardRef` wrapper
+2. Add `ref` to the props destructuring
+3. Update the type to include `ref?: React.Ref<T>`
+
+## Other React 19 Features
+
+- **Actions** — Functions that handle async transitions
+- **useOptimistic** — Optimistic UI updates
+- **useFormStatus** — Form submission state (web)
+- **Document Metadata** — Native `<title>` and `<meta>` support (web)
+
+## Cleanup Checklist
+
+When upgrading to SDK 54:
+
+- [ ] Replace `useContext` with `use`
+- [ ] Remove `.Provider` from Context components
+- [ ] Remove `forwardRef` wrappers, use `ref` prop instead