fix: filter NO_REPLY prefixes

src/agents/system-prompt.ts

@@ -1,4 +1,5 @@
 import type { ThinkLevel } from "../auto-reply/thinking.js";
+import { SILENT_REPLY_TOKEN } from "../auto-reply/tokens.js";
 import type { EmbeddedContextFile } from "./pi-embedded-helpers.js";
 
 export function buildAgentSystemPrompt(params: {
@@ -286,6 +287,18 @@ export function buildAgentSystemPrompt(params: {
   }
 
   lines.push(
+    "## Silent Replies",
+    `When you have nothing to say, respond with ONLY: ${SILENT_REPLY_TOKEN}`,
+    "",
+    "⚠️ Rules:",
+    "- It must be your ENTIRE message — nothing else",
+    `- Never append it to an actual response (never include "${SILENT_REPLY_TOKEN}" in real replies)`,
+    "- Never wrap it in markdown or code blocks",
+    "",
+    `❌ Wrong: "Here's help... ${SILENT_REPLY_TOKEN}"`,
+    `❌ Wrong: "${SILENT_REPLY_TOKEN}"`,
+    `✅ Right: ${SILENT_REPLY_TOKEN}`,
+    "",
     "## Heartbeats",
     heartbeatPromptLine,
     "If you receive a heartbeat poll (a user message matching the heartbeat prompt above), and there is nothing that needs attention, reply exactly:",