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.

<!-- NEXT_JS_LLM_PR -->

packages/next/AGENTS.md

@@ -0,0 +1,7 @@
+<!-- BEGIN:nextjs-agent-rules -->
+
+# This is NOT the Next.js you know
+
+This version has breaking changes — APIs, conventions, and file structure may all differ from your training data. Read the relevant guide in `dist/docs/` before writing any code. Heed deprecation notices.
+
+<!-- END:nextjs-agent-rules -->

packages/next/package.json

@@ -9,6 +9,7 @@
   "homepage": "https://nextjs.org",
   "types": "index.d.ts",
   "files": [
+    "AGENTS.md",
     "dist",
     "app.js",
     "app.d.ts",