fix: pass codex flags through in the stdin-prompt branch

Follow-up to the previous fix: the stdin branch dropped "$@", so
options like -s read-only silently never reached codex. Stub now
records argv and tests assert flags + stdin prompt arrive together;
verified live (-s read-only => codex reports READONLY).

tests/stub-ok/codex

@@ -6,5 +6,5 @@ for a in "$@"; do [[ $prev == "-o" ]] && out=$a; prev=$a; done
 echo '{"type":"thread.started","thread_id":"test-thread-123"}'
 echo '{"type":"turn.completed"}'
 stdin_content=$(cat)
-echo "stub answer|$stdin_content" > "$out"
+echo "stub answer|args=$*|$stdin_content" > "$out"
 exit 0