docs: add Cursor Cloud specific setup instructions to AGENTS.md

Co-authored-by: Piotr Wodecki <PiotrWodecki@users.noreply.github.com>

Author: cursoragent

AGENTS.md

@@ -0,0 +1,10 @@
+# protos
+
+Fishjam protobuf definitions, managed with `buf`.
+
+## Cursor Cloud specific instructions
+
+`buf` is pre-installed (via `mise`). Non-obvious notes:
+
+- Validated: `buf lint`, `buf build`, `buf format` (all pass on `fishjam/`).
+- `prepare.sh` regenerates docs and the Elixir bindings; it uses Docker (`pseudomuto/protoc-gen-doc`) and `fishjam_protos/compile_proto.sh` needs `protoc` with the Elixir plugin plus the Elixir toolchain from `fishjam_protos/.tool-versions`. Those codegen steps are not required just to lint/build the protos.