Heartbeat: inject cron-style current time into prompts (#13733)

* Heartbeat: inject cron-style current time into prompts

* Tests: fix type for web heartbeat timestamp test

* Infra: inline heartbeat current-time injection

src/agents/current-time.ts

@@ -0,0 +1,39 @@
+import {
+  type TimeFormatPreference,
+  formatUserTime,
+  resolveUserTimeFormat,
+  resolveUserTimezone,
+} from "./date-time.js";
+
+export type CronStyleNow = {
+  userTimezone: string;
+  formattedTime: string;
+  timeLine: string;
+};
+
+type TimeConfigLike = {
+  agents?: {
+    defaults?: {
+      userTimezone?: string;
+      timeFormat?: TimeFormatPreference;
+    };
+  };
+};
+
+export function resolveCronStyleNow(cfg: TimeConfigLike, nowMs: number): CronStyleNow {
+  const userTimezone = resolveUserTimezone(cfg.agents?.defaults?.userTimezone);
+  const userTimeFormat = resolveUserTimeFormat(cfg.agents?.defaults?.timeFormat);
+  const formattedTime =
+    formatUserTime(new Date(nowMs), userTimezone, userTimeFormat) ?? new Date(nowMs).toISOString();
+  const timeLine = `Current time: ${formattedTime} (${userTimezone})`;
+  return { userTimezone, formattedTime, timeLine };
+}
+
+export function appendCronStyleCurrentTimeLine(text: string, cfg: TimeConfigLike, nowMs: number) {
+  const base = text.trimEnd();
+  if (!base || base.includes("Current time:")) {
+    return base;
+  }
+  const { timeLine } = resolveCronStyleNow(cfg, nowMs);
+  return `${base}\n${timeLine}`;
+}