Video footage shows Condé Nast employees confronting the company’s head of human resources — part of an incident management characterized as “extreme misconduct” leading to four unionized staffers’ firings — a framing their union rejects. The clips, filmed Wednesday outside HR chief Stan Duncan’s 34th-floor office at One World Trade Center in Lower Manhattan, show about 20 editorial staffers asking to discuss layoffs tied to Teen Vogue’s merger with Vogue.com.   Duncan declines to engage,