The first but obviously the second is implicitly true as well…

Except in Belgium, where “savoir” is often used where one would normally use “pouvoir” 🙂

In my opinion it can often include the idea of “elle avait assez de bon sens pour“, but mostly it means “elle a réussi à“. Ex :

Il a su contrer les attaques de son adversaire avec les réponses appropriées pour s’imposer en finale.

None of your 2 interpretations is fully correct, but IMHO the second is closer. Elle a su from savoir verb referes to knowledge. The first sentence would refer only to wisdom.

My interpretation, is that she knew what to do to find the guards and put them in the right place. The first sentence actually means that she understood that she needed guards which is a fully different question.

But the second sentence only means that she could at the end have the guards, maybe because the were already there, or because someone else has done the job for her, which would not be allowed by the original sentence