Reverts #1116 (80fea00e5). Gemma 4 26B-A4B is served again, from a Nebius
endpoint (staging GEMMA_BASE_URL + GEMMA_API_KEY, AI-1664), and staging's
OpenAI key is invalidated (401 since 22:39:47Z), so Luna-first would fail
every turn before reaching the one model that answers.


Claude-Session: https://claude.ai/code/session_01Jb9F18BsLqrpFePdwtceht

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>