Add skill approval event/response (#12633)

Set the stage for skill-level permission approval in addition to
command-level.

Behind a feature flag.

codex-rs/protocol/src/lib.rs

@@ -15,4 +15,5 @@ pub mod parse_command;
 pub mod plan_tool;
 pub mod protocol;
 pub mod request_user_input;
+pub mod skill_approval;
 pub mod user_input;