Mayor Katie Wilson signed a historic $4 billion agreement Tuesday, marking a turning point in Seattle’s hydropower operations on the Skagit River. It lays out a road map over the next half-century, including investments in salmon passage, habitat and cultural resources.  For over a century, the city of Seattle has “extracted a great deal of value from the Skagit River,” said Chris Townsend, Seattle City Light director of natural resources and hydro licensing. And it is time, Townsend continued, “to return that value.” About half, or $1.8 billion, is for maintenance and operation. The agreement also commits more than a billion dollars to restore salmon and steelhead at risk of extinction, including actions advocated by Native nations for years .…