feat: add comment related to serviceTier logic

Author: NikolaiSviridov

src/CodexAcpClient.ts

@@ -419,6 +419,7 @@ export class CodexAcpClient {
             cwd: null,
             effort: modelSelection.currentReasoningEffort,
             model: modelSelection.currentModelId,
+            // In app-server, explicit null clears the tier; omitting serviceTier would keep the thread's existing tier.
             serviceTier: modelSelection.currentServiceTier,
         });