fix(reply): auto-inject replyToCurrent for reply threading

replyToMode "first"/"all" only filters replyToId but never generates
it — that required the LLM to emit [[reply_to_current]] tags. Inject
replyToCurrent:true on all payloads so applyReplyTagsToPayload sets
replyToId=currentMessageId, then let the existing mode filter decide
which replies keep threading (first only, all, or off).

Covers both final reply path (reply-payloads.ts) and block streaming
path (agent-runner-execution.ts).

src/auto-reply/reply/reply-payloads.auto-threading.test.ts

@@ -0,0 +1,50 @@
+import { describe, expect, it } from "vitest";
+import { applyReplyThreading } from "./reply-payloads.js";
+
+describe("applyReplyThreading auto-injects replyToCurrent", () => {
+  it("sets replyToId to currentMessageId even without [[reply_to_current]] tag", () => {
+    const result = applyReplyThreading({
+      payloads: [{ text: "Hello" }],
+      replyToMode: "first",
+      currentMessageId: "42",
+    });
+
+    expect(result).toHaveLength(1);
+    expect(result[0].replyToId).toBe("42");
+  });
+
+  it("threads only first payload when mode is 'first'", () => {
+    const result = applyReplyThreading({
+      payloads: [{ text: "A" }, { text: "B" }],
+      replyToMode: "first",
+      currentMessageId: "42",
+    });
+
+    expect(result).toHaveLength(2);
+    expect(result[0].replyToId).toBe("42");
+    expect(result[1].replyToId).toBeUndefined();
+  });
+
+  it("threads all payloads when mode is 'all'", () => {
+    const result = applyReplyThreading({
+      payloads: [{ text: "A" }, { text: "B" }],
+      replyToMode: "all",
+      currentMessageId: "42",
+    });
+
+    expect(result).toHaveLength(2);
+    expect(result[0].replyToId).toBe("42");
+    expect(result[1].replyToId).toBe("42");
+  });
+
+  it("strips replyToId when mode is 'off'", () => {
+    const result = applyReplyThreading({
+      payloads: [{ text: "A" }],
+      replyToMode: "off",
+      currentMessageId: "42",
+    });
+
+    expect(result).toHaveLength(1);
+    expect(result[0].replyToId).toBeUndefined();
+  });
+});