Docs: clarify heartbeat active-hours zero-width and 24/7 setup

docs/gateway/heartbeat.md

@@ -163,6 +163,16 @@ Restrict heartbeats to business hours in a specific timezone:
 
 Outside this window (before 9am or after 10pm Eastern), heartbeats are skipped. The next scheduled tick inside the window will run normally.
 
+### 24/7 setup
+
+If you want heartbeats to run all day, use one of these patterns:
+
+- Omit `activeHours` entirely (no time-window restriction).
+- Set a full-day window: `activeHours: { start: "00:00", end: "24:00" }`.
+
+Do not set the same `start` and `end` time (for example `08:00` to `08:00`).
+That is treated as a zero-width window, so heartbeats are always skipped.
+
 ### Multi account example
 
 Use `accountId` to target a specific account on multi-account channels like Telegram:
@@ -214,6 +224,7 @@ Use `accountId` to target a specific account on multi-account channels like Tele
   - Omitted or `"user"`: uses your `agents.defaults.userTimezone` if set, otherwise falls back to the host system timezone.
   - `"local"`: always uses the host system timezone.
   - Any IANA identifier (e.g. `America/New_York`): used directly; if invalid, falls back to the `"user"` behavior above.
+  - `start` and `end` must not be equal for an active window; equal values are treated as zero-width (always outside the window).
   - Outside the active window, heartbeats are skipped until the next tick inside the window.
 
 ## Delivery behavior