feat(date-time): standardize time context and tool timestamps

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

@@ -5,7 +5,7 @@ import { enqueueSystemEvent, resetSystemEventsForTest } from "../../infra/system
 import { prependSystemEvents } from "./session-updates.js";
 
 describe("prependSystemEvents", () => {
-  it("adds a local timestamp to queued system events", async () => {
+  it("adds a UTC timestamp to queued system events", async () => {
     vi.useFakeTimers();
     const timestamp = new Date("2026-01-12T20:19:17");
     vi.setSystemTime(timestamp);
@@ -20,15 +20,7 @@ describe("prependSystemEvents", () => {
       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",
-    });
+    const expectedTimestamp = "2026-01-12T20:19:17Z";
 
     expect(result).toContain(`System: [${expectedTimestamp}] Model switched.`);