[codex] document out-of-line test module convention

AGENTS.md

@@ -110,6 +110,19 @@ See `codex-rs/tui/styles.md`.
 
 ## Tests
 
+### Test module organization
+
+- When adding a new test module, define its contents in a separate sibling file rather than inline in the implementation file.
+- Use an explicit `#[path = "..._tests.rs"]` attribute so the test filename is descriptive and easy to locate:
+
+  ```rust
+  #[cfg(test)]
+  #[path = "parser_tests.rs"]
+  mod tests;
+  ```
+
+- This applies only when introducing a new test module. Do not move or rewrite existing inline `#[cfg(test)] mod tests { ... }` modules solely to follow this convention.
+
 ### Snapshot tests
 
 This repo uses snapshot tests (via `insta`), especially in `codex-rs/tui`, to validate rendered output.