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/core/tests/suite/mod.rs

@@ -111,6 +111,7 @@ mod seatbelt;
 mod shell_command;
 mod shell_serialization;
 mod shell_snapshot;
+mod skill_approval;
 mod skills;
 mod sqlite_state;
 mod stream_error_allows_next_turn;