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/app-server/tests/suite/v2/mod.rs

@@ -16,6 +16,7 @@ mod rate_limits;
 mod request_user_input;
 mod review;
 mod safety_check_downgrade;
+mod skill_approval;
 mod skills_list;
 mod thread_archive;
 mod thread_fork;