System events: add local timestamps in prompt injection

Closes #245

src/auto-reply/reply/session-updates.test.ts

@@ -0,0 +1,43 @@
+import { describe, expect, it, vi } from "vitest";
+
+import type { ClawdbotConfig } from "../../config/config.js";
+import {
+  enqueueSystemEvent,
+  resetSystemEventsForTest,
+} from "../../infra/system-events.js";
+import { prependSystemEvents } from "./session-updates.js";
+
+describe("prependSystemEvents", () => {
+  it("adds a local timestamp to queued system events", async () => {
+    vi.useFakeTimers();
+    const timestamp = new Date("2026-01-12T20:19:17");
+    vi.setSystemTime(timestamp);
+
+    enqueueSystemEvent("Model switched.", { sessionKey: "agent:main:main" });
+
+    const result = await prependSystemEvents({
+      cfg: {} as ClawdbotConfig,
+      sessionKey: "agent:main:main",
+      isMainSession: false,
+      isNewSession: false,
+      prefixedBodyBase: "User: hi",
+    });
+
+    const expectedTimestamp = timestamp.toLocaleString("en-US", {
+      hour12: false,
+      year: "numeric",
+      month: "2-digit",
+      day: "2-digit",
+      hour: "2-digit",
+      minute: "2-digit",
+      second: "2-digit",
+    });
+
+    expect(result).toContain(
+      `System: [${expectedTimestamp}] Model switched.`,
+    );
+
+    resetSystemEventsForTest();
+    vi.useRealTimers();
+  });
+});