Detect Codex in is_llm() (#2352)

Fixes #2317

Author: hadley

R/reporter-llm.R

@@ -6,7 +6,7 @@
 #' occur and the total number of successes at the end.
 #'
 #' `LlmReporter` is used by default when tests are run by a coding agent.
-#' Currently we detect Claude Code, Cursor, and Gemini CLI.
+#' Currently we detect Claude Code, Codex, Cursor, and Gemini CLI.
 #' If using another tool, configure it to set env var `AGENT=1`.
 #'
 #' @export
@@ -91,5 +91,6 @@ is_llm <- function() {
   nzchar(Sys.getenv("AGENT")) ||
     nzchar(Sys.getenv("CLAUDECODE")) ||
     nzchar(Sys.getenv("GEMINI_CLI")) ||
-    nzchar(Sys.getenv("CURSOR_AGENT"))
+    nzchar(Sys.getenv("CURSOR_AGENT")) ||
+    nzchar(Sys.getenv("CODEX_CI"))
 }