fix(cron): make --tz work with --at for one-shot jobs

Previously, `--at` with an offset-less ISO datetime (e.g. `2026-03-23T23:00:00`)
was always interpreted as UTC, even when `--tz` was provided. This caused one-shot
jobs to fire at the wrong time.

Changes:
- `parseAt()` now accepts an optional `tz` parameter
- When `--tz` is provided with `--at`, offset-less datetimes are interpreted in
  that IANA timezone using Intl.DateTimeFormat
- Datetimes with explicit offsets (e.g. `+01:00`, `Z`) are unaffected
- Removed the guard in cron-edit that blocked `--tz` with `--at`
- Updated `--at` help text to mention `--tz` support
- Added 2 tests verifying timezone resolution and offset preservation

src/cli/cron-cli/register.cron-add.ts

@@ -73,7 +73,10 @@ export function registerCronAddCommand(cron: Command) {
       .option("--session <target>", "Session target (main|isolated)")
       .option("--session-key <key>", "Session key for job routing (e.g. agent:my-agent:my-session)")
       .option("--wake <mode>", "Wake mode (now|next-heartbeat)", "now")
-      .option("--at <when>", "Run once at time (ISO) or +duration (e.g. 20m)")
+      .option(
+        "--at <when>",
+        "Run once at time (ISO with offset, or +duration). Use --tz for offset-less datetimes",
+      )
       .option("--every <duration>", "Run every duration (e.g. 10m, 1h)")
       .option("--cron <expr>", "Cron expression (5-field or 6-field with seconds)")
       .option("--tz <iana>", "Timezone for cron expressions (IANA)", "")
@@ -119,7 +122,9 @@ export function registerCronAddCommand(cron: Command) {
               throw new Error("--stagger/--exact are only valid with --cron");
             }
             if (at) {
-              const atIso = parseAt(at);
+              const tzRaw =
+                typeof opts.tz === "string" && opts.tz.trim() ? opts.tz.trim() : undefined;
+              const atIso = parseAt(at, tzRaw);
               if (!atIso) {
                 throw new Error("Invalid --at; use ISO time or duration like 20m");
               }

src/cli/cron-cli/register.cron-edit.ts

@@ -158,14 +158,13 @@ export function registerCronEditCommand(cron: Command) {
           if (scheduleChosen > 1) {
             throw new Error("Choose at most one schedule change");
           }
-          if (
-            (requestedStaggerMs !== undefined || typeof opts.tz === "string") &&
-            (opts.at || opts.every)
-          ) {
-            throw new Error("--stagger/--exact/--tz are only valid for cron schedules");
+          if (requestedStaggerMs !== undefined && (opts.at || opts.every)) {
+            throw new Error("--stagger/--exact are only valid for cron schedules");
           }
           if (opts.at) {
-            const atIso = parseAt(String(opts.at));
+            const tzRaw =
+              typeof opts.tz === "string" && opts.tz.trim() ? opts.tz.trim() : undefined;
+            const atIso = parseAt(String(opts.at), tzRaw);
             if (!atIso) {
               throw new Error("Invalid --at");
             }

src/cli/cron-cli/shared.ts

@@ -89,11 +89,38 @@ export function parseCronStaggerMs(params: {
   return parsed;
 }
 
-export function parseAt(input: string): string | null {
+/**
+ * Parse a one-shot `--at` value into an ISO string (UTC).
+ *
+ * When `tz` is provided and the input is an offset-less datetime
+ * (e.g. `2026-03-23T23:00:00`), the datetime is interpreted in
+ * that IANA timezone instead of UTC.
+ */
+export function parseAt(input: string, tz?: string): string | null {
   const raw = input.trim();
   if (!raw) {
     return null;
   }
+
+  // If a timezone is provided and the input looks like an offset-less ISO datetime,
+  // resolve it in the given IANA timezone so users get the time they expect.
+  if (tz) {
+    const isoNoOffset = /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}(:\d{2})?(\.\d+)?$/.test(raw);
+    if (isoNoOffset) {
+      try {
+        // Use Intl to find the UTC offset for the given tz at the specified local time.
+        // We first parse naively as UTC to get a rough Date, then compute the real offset.
+        const naiveMs = new Date(`${raw}Z`).getTime();
+        if (!Number.isNaN(naiveMs)) {
+          const offset = getTimezoneOffsetMs(naiveMs, tz);
+          return new Date(naiveMs - offset).toISOString();
+        }
+      } catch {
+        // Fall through to default parsing if tz is invalid
+      }
+    }
+  }
+
   const absolute = parseAbsoluteTimeMs(raw);
   if (absolute !== null) {
     return new Date(absolute).toISOString();
@@ -105,6 +132,42 @@ export function parseAt(input: string): string | null {
   return null;
 }
 
+/**
+ * Get the UTC offset in milliseconds for a given IANA timezone at a given UTC instant.
+ * Positive means ahead of UTC (e.g. +3600000 for CET).
+ */
+function getTimezoneOffsetMs(utcMs: number, tz: string): number {
+  const d = new Date(utcMs);
+  // Format parts in the target timezone
+  const parts = new Intl.DateTimeFormat("en-US", {
+    timeZone: tz,
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+    hour: "2-digit",
+    minute: "2-digit",
+    second: "2-digit",
+    hour12: false,
+  }).formatToParts(d);
+
+  const get = (type: string) => {
+    const part = parts.find((p) => p.type === type);
+    return Number.parseInt(part?.value ?? "0", 10);
+  };
+
+  // Reconstruct the local time as if it were UTC
+  const localAsUtc = Date.UTC(
+    get("year"),
+    get("month") - 1,
+    get("day"),
+    get("hour") === 24 ? 0 : get("hour"),
+    get("minute"),
+    get("second"),
+  );
+
+  return localAsUtc - utcMs;
+}
+
 const CRON_ID_PAD = 36;
 const CRON_NAME_PAD = 24;
 const CRON_SCHEDULE_PAD = 32;