feat: codex-run.sh watchdog wrapper + stub tests

tests/stub-ok/codex

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# fake codex: emits thread.started, writes last message file, exits 0
+out=""
+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"
+exit 0