AI: harness-aware model validation, official codex contract, launch fail-fast

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QKxGW1raAWaqeU8WdHsMsp

bin/omarchy-cn-ai-launch

@@ -37,7 +37,8 @@ env)
     cn_ai_render_codex_config "$provider" "$model" "$key"
     exec "$command"
   fi
-  eval "$(cn_ai_render_env "$harness" "$provider" "$model" "$key")"
+  env_exports=$(cn_ai_render_env "$harness" "$provider" "$model" "$key")
+  eval "$env_exports"
   exec "$command"
   ;;
 native)