fix(agent): add NO_REPLY detection utility for filtering silent replies

Extract SILENT_REPLY_TOKEN and isSilentReplyText() into a shared
module. Detects NO_REPLY at the start or end of text (with optional
whitespace/punctuation) to filter out silent announcement responses
that should not be forwarded to the user.

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

packages/core/src/agent/tokens.test.ts

@@ -0,0 +1,39 @@
+import { describe, it, expect } from "vitest";
+import { isSilentReplyText, SILENT_REPLY_TOKEN } from "./tokens.js";
+
+describe("isSilentReplyText", () => {
+  it("detects exact NO_REPLY", () => {
+    expect(isSilentReplyText("NO_REPLY")).toBe(true);
+  });
+
+  it("detects NO_REPLY with surrounding whitespace", () => {
+    expect(isSilentReplyText("  NO_REPLY  ")).toBe(true);
+    expect(isSilentReplyText("\nNO_REPLY\n")).toBe(true);
+  });
+
+  it("detects NO_REPLY with trailing punctuation", () => {
+    expect(isSilentReplyText("NO_REPLY.")).toBe(true);
+    expect(isSilentReplyText("NO_REPLY.\n")).toBe(true);
+  });
+
+  it("detects NO_REPLY at end of text", () => {
+    expect(isSilentReplyText("I have nothing to report. NO_REPLY")).toBe(true);
+  });
+
+  it("returns false for undefined/empty", () => {
+    expect(isSilentReplyText(undefined)).toBe(false);
+    expect(isSilentReplyText("")).toBe(false);
+  });
+
+  it("returns false for normal text", () => {
+    expect(isSilentReplyText("Here are the findings")).toBe(false);
+  });
+
+  it("returns false for NO_REPLY embedded in a word", () => {
+    expect(isSilentReplyText("DONO_REPLYX")).toBe(false);
+  });
+
+  it("exports SILENT_REPLY_TOKEN as NO_REPLY", () => {
+    expect(SILENT_REPLY_TOKEN).toBe("NO_REPLY");
+  });
+});

packages/core/src/agent/tokens.ts

@@ -0,0 +1,17 @@
+export const SILENT_REPLY_TOKEN = "NO_REPLY";
+
+function escapeRegExp(value: string): string {
+  return value.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+}
+
+export function isSilentReplyText(
+  text: string | undefined,
+  token: string = SILENT_REPLY_TOKEN,
+): boolean {
+  if (!text) return false;
+  const escaped = escapeRegExp(token);
+  const prefix = new RegExp(`^\\s*${escaped}(?=$|\\W)`);
+  if (prefix.test(text)) return true;
+  const suffix = new RegExp(`\\b${escaped}\\b\\W*$`);
+  return suffix.test(text);
+}