fix(discord): replyToMode first behaviour

src/discord/monitor/threading.test.ts

@@ -93,7 +93,22 @@ describe("resolveDiscordReplyDeliveryPlan", () => {
       threadChannel: { id: "thread" },
       createdThreadId: null,
     });
+    // "all" returns the reference on every call.
     expect(plan.replyReference.use()).toBe("m1");
+    expect(plan.replyReference.use()).toBe("m1");
+  });
+
+  it("uses existingId only on first call with replyToMode first inside a thread", () => {
+    const plan = resolveDiscordReplyDeliveryPlan({
+      replyTarget: "channel:thread",
+      replyToMode: "first",
+      messageId: "m1",
+      threadChannel: { id: "thread" },
+      createdThreadId: null,
+    });
+    // "first" returns the reference only once.
+    expect(plan.replyReference.use()).toBe("m1");
+    expect(plan.replyReference.use()).toBeUndefined();
   });
 });