docs: add comment explaining stream:true hardcoding in responses route

Codex API only supports streaming, so the proxy always sends stream:true
upstream and uses collect mode to assemble JSON for non-streaming clients.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/routes/responses.ts

@@ -218,6 +218,8 @@ export function createResponsesRoutes(
     const modelInfo = getModelInfo(modelId);
 
     // Build CodexResponsesRequest
+    // Codex API only supports streaming — stream/store are always true/false.
+    // When client sends stream:false, the proxy collects SSE events and returns assembled JSON.
     const codexRequest: CodexResponsesRequest = {
       model: modelId,
       instructions: body.instructions,