fix: heartbeat prompt + dedupe (#980) (thanks @voidserf)

- tighten default heartbeat prompt guidance
- suppress duplicate heartbeat alerts within 24h

Co-authored-by: void <voidserf@users.noreply.github.com>

src/auto-reply/heartbeat.ts

@@ -1,7 +1,9 @@
 import { HEARTBEAT_TOKEN } from "./tokens.js";
 
+// Default heartbeat prompt (used when config.agents.defaults.heartbeat.prompt is unset).
+// Keep it tight and avoid encouraging the model to invent/rehash "open loops" from prior chat context.
 export const HEARTBEAT_PROMPT =
-  "Consider outstanding tasks and HEARTBEAT.md guidance from the workspace context (if present). Checkup sometimes on your human during (user local) day time.";
+  "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.";
 export const DEFAULT_HEARTBEAT_EVERY = "30m";
 export const DEFAULT_HEARTBEAT_ACK_MAX_CHARS = 300;