feat: grep_files as a tool (#4820)

Add `grep_files` to be able to perform more action in parallel

codex-rs/core/tests/suite/mod.rs

@@ -9,6 +9,7 @@ mod compact_resume_fork;
 mod exec;
 mod exec_stream_events;
 mod fork_conversation;
+mod grep_files;
 mod json_result;
 mod list_dir;
 mod live_cli;