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/config/sessions/types.ts

@@ -10,6 +10,13 @@ export type SessionChannelId = ChannelId | "webchat";
 export type SessionChatType = "direct" | "group" | "room";
 
 export type SessionEntry = {
+  /**
+   * Last delivered heartbeat payload (used to suppress duplicate heartbeat notifications).
+   * Stored on the main session entry.
+   */
+  lastHeartbeatText?: string;
+  /** Timestamp (ms) when lastHeartbeatText was delivered. */
+  lastHeartbeatSentAt?: number;
   sessionId: string;
   updatedAt: number;
   sessionFile?: string;