Broadly, the global or system goal of Lean Thinking is to go from “concept to cash” as fast as possible at a sustainable pace, which means quickly deliver things of value in shorter and shorter cycle times of all processes while still achieving highest quality and morale levels.
Agile is a quality of the organization and its people to be adaptive, responsive, continually learning and evolving to be agile, with the goal of competitive business success and rapid delivery of economically valuable products and knowledge.
Webster dictionary defines Agile as ready ability to move with quick easy grace.
Agile does not mean delivering faster, it does not mean fewer defects and higher quality, it does not mean higher productivity either. Agile means agile. It means to be nimble and adaptive. Be Agile means to embrace change and become master of change. It means to be able to change faster than your competitor can. This agility is supported by both lean and agile practices.
Obviously with Agile methods such as Scrum or XP an organization could achieve faster delivery and higher quality, however the main idea of agile methods is agility.
Therefore Agile cannot be merged to Lean by definition, rather enterprises will be applying lean thinking together with systems thinking and queuing theory to adopt and enhance concrete agile methods and become Agile.