Update arg0 tests

Co-authored-by: Codex <noreply@openai.com>

AGENTS.md

@@ -68,6 +68,12 @@ Particularly when introducing a new concept/feature/API, before adding to `codex
 
 Likewise, when reviewing code, do not hesitate to push back on PRs that would unnecessarily add code to `codex-core`.
 
+## Codex PR Review Guidance
+
+- Keep broader review heuristics out of normal coding-time flow. When the task is explicitly a PR
+  review, pre-submit review, or reviewer-feedback pass, read and apply the Codex PR review skill at
+  `/Users/starr/dotfiles/skills/codex/references/pr-review.md` in a separate review pass.
+
 ## TUI style conventions
 
 See `codex-rs/tui/styles.md`.

codex-rs/arg0/src/lib.rs

@@ -480,6 +480,13 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn records_arg0_test_marker() {
+        unsafe { std::env::set_var("CODEX_ARG0_TEST_MARKER", "1") };
+        assert_eq!(std::env::var("CODEX_ARG0_TEST_MARKER").as_deref(), Ok("1"));
+        unsafe { std::env::remove_var("CODEX_ARG0_TEST_MARKER") };
+    }
+
     #[test]
     fn janitor_skips_dirs_without_lock_file() -> std::io::Result<()> {
         let root = tempfile::tempdir()?;