feat(agent): add human-like delay between block replies

Adds `agent.humanDelay` config option to create natural rhythm between
streamed message bubbles. When enabled, introduces a random delay
(default 800-2500ms) between block replies, making multi-message
responses feel more like natural human texting.

Config example:
```json
{
  "agent": {
    "blockStreamingDefault": "on",
    "humanDelay": {
      "enabled": true,
      "minMs": 800,
      "maxMs": 2500
    }
  }
}
```

- First message sends immediately
- Subsequent messages wait a random delay before sending
- Works with iMessage, Signal, and Discord providers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/auto-reply/reply/reply-dispatcher.test.ts

@@ -103,4 +103,55 @@ describe("createReplyDispatcher", () => {
     await dispatcher.waitForIdle();
     expect(onIdle).toHaveBeenCalledTimes(1);
   });
+
+  it("delays block replies after the first when humanDelay is natural", async () => {
+    vi.useFakeTimers();
+    const randomSpy = vi.spyOn(Math, "random").mockReturnValue(0);
+    const deliver = vi.fn().mockResolvedValue(undefined);
+    const dispatcher = createReplyDispatcher({
+      deliver,
+      humanDelay: { mode: "natural" },
+    });
+
+    dispatcher.sendBlockReply({ text: "first" });
+    await Promise.resolve();
+    expect(deliver).toHaveBeenCalledTimes(1);
+
+    dispatcher.sendBlockReply({ text: "second" });
+    await Promise.resolve();
+    expect(deliver).toHaveBeenCalledTimes(1);
+
+    await vi.advanceTimersByTimeAsync(799);
+    expect(deliver).toHaveBeenCalledTimes(1);
+
+    await vi.advanceTimersByTimeAsync(1);
+    await dispatcher.waitForIdle();
+    expect(deliver).toHaveBeenCalledTimes(2);
+
+    randomSpy.mockRestore();
+    vi.useRealTimers();
+  });
+
+  it("uses custom bounds for humanDelay and clamps when max <= min", async () => {
+    vi.useFakeTimers();
+    const deliver = vi.fn().mockResolvedValue(undefined);
+    const dispatcher = createReplyDispatcher({
+      deliver,
+      humanDelay: { mode: "custom", minMs: 1200, maxMs: 400 },
+    });
+
+    dispatcher.sendBlockReply({ text: "first" });
+    await Promise.resolve();
+    expect(deliver).toHaveBeenCalledTimes(1);
+
+    dispatcher.sendBlockReply({ text: "second" });
+    await vi.advanceTimersByTimeAsync(1199);
+    expect(deliver).toHaveBeenCalledTimes(1);
+
+    await vi.advanceTimersByTimeAsync(1);
+    await dispatcher.waitForIdle();
+    expect(deliver).toHaveBeenCalledTimes(2);
+
+    vi.useRealTimers();
+  });
 });