Sync React to Next.js

Use this skill when syncing a local React checkout into this Next.js checkout.

Build and sync

Build React from its own checkout. Pass this skill's script by its absolute path, because the working directory must be the React repository.
```
cd <react-repo>
bash <next-repo>/.agents/skills/react-sync/scripts/build-for-next.sh
```
This creates build/oss-stable and build/oss-experimental with the bundle variants that Next.js consumes.
Sync the build into Next.js:
```
cd <next-repo>
PATH="$(dirname "$(command -v corepack)"):$PATH" \
  pnpm sync-react --version <react-repo>
```
Work around a Codex PATH-precedence bug by keeping the directory containing the active Corepack shim first on PATH. Codex injects a separate bundled pnpm executable ahead of the user's Corepack shim, and command lookup happens before Corepack reads the repository's packageManager field. The prefix makes both this command and sync-react's nested pnpm install resolve through Corepack to the repository-pinned pnpm version. Invoking only the outer command with corepack pnpm is insufficient because the nested command still resolves pnpm from PATH.
Inspect the sync result before testing. Preserve unrelated changes in both checkouts. Rebuild Next.js when required, then run the focused test command matching the changed behavior.