K^2-Agent: Co-Evolving Know-What and Know-How for Hierarchical Mobile Device Control - Wu, Zhe; Mo, Donglin; Lu, Hongjin; Xing, Junliang; Liu, Jianheng; Jing, Yuheng; Li, Kai; Shao, Kun; Hao, Jianye; Shi, Yuanchun (International Conference on Learning Representations, 2026)