Add React sync development skill (#95620)
### What?
Add a repository skill for building local React stable and experimental
bundles, syncing them into Next.js, and verifying the integration.
### Why?
Testing upstream React changes in Next.js requires a precise bundle
list, both React release channels, and the repository-pinned pnpm
version. Capturing the workflow as a skill makes it repeatable and
avoids global pnpm versions bypassing workspace linking.
### How?
Add `$react-sync` with a reusable build helper and document it in
`AGENTS.md`. The sync command puts the Corepack shim first on `PATH` so
nested pnpm commands use the version pinned by Next.js.
### Verification
- `bash -n .agents/skills/react-sync/scripts/build-for-next.sh`
- `python3
~/.codex/skills/.system/skill-creator/scripts/quick_validate.py
.agents/skills/react-sync`
- Prettier check for `AGENTS.md` and `SKILL.md`
- Full local React build and `pnpm sync-react` workflow completed
successfully
- `pnpm --filter=next build`
<!-- NEXT_JS_LLM_PR -->
