fix: prompt.md via stdin now combines with codex flags

Options like -s read-only routed execution into the /dev/null branch,
so codex got no prompt at all. codex natively appends piped stdin as
a <stdin> block, so feeding prompt.md unconditionally is safe.

tests/stub-ok/codex

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