South Sudan has moved troops into Sudan’s Heglig oil field under what it calls a tripartite agreement with Sudan’s two warring factions. The move is a rare arrangement aimed at shielding critical oil infrastructure as fighting escalates across West Kordofan. South Sudan’s army chief of staff, Paul Nang, appeared in a video address from Heglig, saying South Sudanese forces entered the field following an agreement between President Salva Kiir, Sudanese Armed Forces leader Abdelfattah El Burhan, and Rapid Support Forces commander…