Set user-agent (#2230)

Use the same well-defined value in all cases when sending user-agent
header

codex-rs/core/src/lib.rs

@@ -47,6 +47,7 @@ pub mod seatbelt;
 pub mod shell;
 pub mod spawn;
 pub mod turn_diff_tracker;
+pub mod user_agent;
 mod user_notification;
 pub mod util;
 pub use apply_patch::CODEX_APPLY_PATCH_ARG1;