fix: add AGENTS.md at package root for agent discovery (#92376)
### What?
Add an `AGENTS.md` file at the npm package root
(`node_modules/next/AGENTS.md`) so AI agents can discover the bundled
documentation.
### Why?
AI coding agents naturally check the package root for `AGENTS.md` before
searching subdirectories. Without a file at the root, `cat
node_modules/next/AGENTS.md` fails and agents hit a dead end. The
bundled docs at `dist/docs/` are already shipped with the package but
are not discoverable from the root.
### How?
- Add `packages/next/AGENTS.md` mirroring the same content
`create-next-app` generates for the project root, with the path adjusted
to point at `dist/docs/` relative to the package root.
- Add `"AGENTS.md"` to the `files` array in `package.json` so it ships
with the npm package.
### Improving Documentation
- [x] Run `pnpm prettier-fix` to fix formatting issues before opening
the PR.
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