As you suggest, “les travaux (…) d’ores et déjà effectués” refers to the work
done before the contract was signed. However, it doesn’t refer to everything
the “salarié” has ever done, since this is later qualified by “pour le compte et
sous la direction de X et dans le cadre de son contrat de travail”. Thus, only
the work done under X’s supervision will belong to X, as is usual.