Mayor-elect Zohran Mamdani has tapped a 74-year-old former high-ranking de Blasio administration official and longtime budget guru as his top City Hall adviser. The appointment of Dean Fuleihan as first deputy mayor marks the first major staffing decision for the incoming Mamdani administration that wants to spend more than $10 billion on a lofty socialist agenda while staring at a nearly $5 billion budget gap. Fuleihan, who is more than twice the age of the 34-year-old socialist lawmaker, also served as then-Mayor Bill de Blasio’s first deputy mayor and had been advising Mamdani on his campaign.