feat(heartbeat): improve prompting, suppression, delivery targets and session cleanup

- Rewrite heartbeat prompt to instruct agent to EXECUTE checklist tasks, not echo them
- Simplify suppression: HEARTBEAT_OK present = always suppress, absent = always deliver
- Add delivery targets RPC (heartbeat.targets) for channel/chatId picker
- Sanitize backend errors — never expose raw SQL to client
- Add session cleanup for isolated heartbeat sessions after run
- Cap StaggerOffset at 10% of interval to avoid user-visible delay
- Fix Upsert to persist next_run_at correctly

cmd/gateway.go

@@ -711,6 +711,7 @@ func runGateway() {
 	heartbeatTicker := heartbeat.NewTicker(heartbeat.TickerConfig{
 		Store:    pgStores.Heartbeats,
 		Agents:   pgStores.Agents,
+		Sessions: pgStores.Sessions,
 		MsgBus:   msgBus,
 		Sched:    sched,
 		RunAgent: makeHeartbeatRunFn(sched),