DAMASCUS, SYRIA — Syria’s central government has reached a deal with the Kurdish-led authority that controls the country’s northeast, including a ceasefire and the merging of the main U.S.-backed force there into the Syrian army. The deal was signed Monday by interim President Ahmad al-Sharaa and Mazloum Abdi, the commander of the U.S.-backed, Kurdish-led Syrian Democratic Forces. The deal marks a major breakthrough that would bring most of Syria under the control of the government led by the group that led the ousting of President Bashar al-Assad in December. The deal to be implemented by the end of the year