docs: Add AGENTS.md for Next.js agent rules and reference in CLAUDE.md

* Introduced AGENTS.md to outline breaking changes and guidelines for the Next.js agent, emphasizing the differences from previous versions.
* Updated CLAUDE.md to reference AGENTS.md, ensuring documentation consistency and accessibility for users.

Author: cevheri

AGENTS.md

@@ -0,0 +1,9 @@
+<!-- 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 `node_modules/next/dist/docs/` (resolved from this file's directory; in monorepos the `next` package may not be visible from the repo root) before writing any code. Heed deprecation notices.
+
+This block is written and re-added by `next dev` — verify at `node_modules/next/dist/server/lib/generate-agent-files.js`. Removing it from a diff only re-creates the uncommitted change; committing it with your work keeps the tree clean.
+
+<!-- END:nextjs-agent-rules -->

CLAUDE.md

@@ -158,3 +158,6 @@ only `INSERT` raises, which is why that suite has both `expectRefused` and
 | [`docs/manager-queries.md`](docs/manager-queries.md) | Tables, columns, joins, manager SQL |
 | [`SECURITY.md`](SECURITY.md) | The shared-origin exposure, accepted risks, operational rules |
 | [`supabase/README.md`](supabase/README.md) | Applying the schema |
+
+
+@AGENTS.md