MOGADISHU, Somalia — Another diplomatic thaw was underway Saturday in the restless Horn of Africa region as Somalia’s president visited Eritrea for the first time and years of tension gave way to an embrace. “Somalia is ready to write a new chapter of its relations with Eritrea,” Abdinur Mohamed, a spokesman for President Mohamed Abdullahi Mohamed, announced on Twitter. The two nations have not had diplomatic ties for nearly 15 years.