fix: keep Slack thread replies in thread

src/slack/monitor.tool-result.test.ts

@@ -122,4 +122,38 @@ describe("monitorSlackProvider tool results", () => {
     expect(sendMock.mock.calls[0][1]).toBe("PFX tool update");
     expect(sendMock.mock.calls[1][1]).toBe("PFX final reply");
   });
+
+  it("threads replies when incoming message is in a thread", async () => {
+    replyMock.mockResolvedValue({ text: "thread reply" });
+
+    const controller = new AbortController();
+    const run = monitorSlackProvider({
+      botToken: "bot-token",
+      appToken: "app-token",
+      abortSignal: controller.signal,
+    });
+
+    await waitForEvent("message");
+    const handler = getSlackHandlers()?.get("message");
+    if (!handler) throw new Error("Slack message handler not registered");
+
+    await handler({
+      event: {
+        type: "message",
+        user: "U1",
+        text: "hello",
+        ts: "123",
+        thread_ts: "456",
+        channel: "C1",
+        channel_type: "im",
+      },
+    });
+
+    await flush();
+    controller.abort();
+    await run;
+
+    expect(sendMock).toHaveBeenCalledTimes(1);
+    expect(sendMock.mock.calls[0][2]).toMatchObject({ threadTs: "456" });
+  });
 });