feat: add per-agent heartbeat config

docs/gateway/configuration.md

@@ -1675,9 +1675,14 @@ Z.AI models are available as `zai/<model>` (e.g. `zai/glm-4.7`) and require
 - `includeReasoning`: when `true`, heartbeats will also deliver the separate `Reasoning:` message when available (same shape as `/reasoning on`). Default: `false`.
 - `target`: optional delivery channel (`last`, `whatsapp`, `telegram`, `discord`, `slack`, `signal`, `imessage`, `none`). Default: `last`.
 - `to`: optional recipient override (channel-specific id, e.g. E.164 for WhatsApp, chat id for Telegram).
-- `prompt`: optional override for the heartbeat body (default: `Read HEARTBEAT.md if exists. Consider outstanding tasks. Checkup sometimes on your human during (user local) day time.`). Overrides are sent verbatim; include a `Read HEARTBEAT.md if exists` line if you still want the file read.
+- `prompt`: optional override for the heartbeat body (default: `Read HEARTBEAT.md if it exists (workspace context). Follow it strictly. Do not infer or repeat old tasks from prior chats. If nothing needs attention, reply HEARTBEAT_OK.`). Overrides are sent verbatim; include a `Read HEARTBEAT.md` line if you still want the file read.
 - `ackMaxChars`: max chars allowed after `HEARTBEAT_OK` before delivery (default: 300).
 
+Per-agent heartbeats:
+- Set `agents.list[].heartbeat` to enable or override heartbeat settings for a specific agent.
+- If any agent entry defines `heartbeat`, **only those agents** run heartbeats; defaults
+  become the shared baseline for those agents.
+
 Heartbeats run full agent turns. Shorter intervals burn more tokens; be mindful
 of `every`, keep `HEARTBEAT.md` tiny, and/or choose a cheaper `model`.