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

pkg/protocol/methods.go

@@ -92,6 +92,7 @@ const (
 	MethodHeartbeatLogs         = "heartbeat.logs"
 	MethodHeartbeatChecklistGet = "heartbeat.checklist.get"
 	MethodHeartbeatChecklistSet = "heartbeat.checklist.set"
+	MethodHeartbeatTargets      = "heartbeat.targets"
 )
 
 // Channel instances management