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

docs/CONFIG.example.json

@@ -22,7 +22,8 @@
           "debugLogChannelIds": [],
           "discordControlApiBaseUrl": "http://127.0.0.1:8790",
           "discordControlApiToken": "<DISCORD_CONTROL_AUTH_TOKEN>",
-          "discordControlCallerId": "agent-main"
+          "discordControlCallerId": "agent-main",
+          "moderatorBotToken": "<MODERATOR_BOT_TOKEN>"
         }
       }
     }