fix: use systemPrompt instead of prependContext for end marker instruction

- Change before_prompt_build hook to return systemPrompt instead of prependContext
- This ensures the end marker instruction is injected once per session as a system prompt, not repeatedly prepended to each user message
- Add moderatorBotToken to CONFIG.example.json for documentation

plugin/index.ts

@@ -726,8 +726,8 @@ export default {
         if (idStr) identity = idStr + "\n\n";
       }
 
-      api.logger.info(`whispergate: prepend end marker instruction for session=${key}, reason=${rec.decision.reason} isGroupChat=${isGroupChat}`);
-      return { prependContext: identity + instruction };
+      api.logger.info(`whispergate: set system prompt for session=${key}, reason=${rec.decision.reason} isGroupChat=${isGroupChat}`);
+      return { systemPrompt: identity + instruction };
     });
 
     // Register slash commands for Discord