Add Elixir style rule with code example

AGENTS.md

@@ -1,6 +1,6 @@
 This is a web application written using the Phoenix web framework.
 
-## Project guidelines
+## Project Overview
 
 - **Always** run unit tests with `mix test.unit` and autoformat with `mix format` after making edits
 - Use `mix precommit` alias when you are done with all changes and fix any pending issues
@@ -46,6 +46,44 @@ This is a Nostr relay built with Phoenix 1.8 / Elixir. It stores and serves Nost
 
 REST API at `/api/events` (show, create, delete) with JSON rendering. Uses `FallbackController` for error handling.
 
+## Development Guidelines
+
+### Elixir Code Style Rules
+
+Avoid excessively nested expressions. Prefer named variables to incrementally store results. Example:
+
+```elixir
+# BAD: Deeply nested result expression
+def query_events(filter_map) do
+  with {:ok, filter} <- Filter.from_map(filter_map) do
+    events =
+      from(e in Event)
+      |> Filter.apply(filter)
+
+    {:ok,
+     Enum.map(events, fn event ->
+       {:ok, pub_event} = PubEvent.from_db(event)
+       pub_event
+     end)}
+  end
+end
+
+# GOOD: Build the result incrementally with named variables
+def query_events(filter_map) when is_map(filter_map) do
+  with {:ok, filter} <- Filter.from_map(filter_map),
+       events <-
+         from(e in Event)
+         |> Filter.apply(filter),
+       pub_events <-
+         Enum.map(events, fn event ->
+           {:ok, pub_event} = PubEvent.from_db(event)
+           pub_event
+         end) do
+    {:ok, pub_events}
+  end
+end
+```
+
 ### Test Structure
 
 - Tests tagged `@moduletag :unit` run with `mix test.unit` (no DB).