Bob Weir, the unassuming singer, songwriter and nuanced rhythm guitarist known as the “other” founding member of the Grateful Dead, has died, his family announced Saturday on Instagram. He was 78. Weir “transitioned peacefully, surrounded by loved ones, after courageously beating cancer as only Bobby could. Unfortunately, he succumbed to underlying lung issues,” they said. He was diagnosed in July but returned to his hometown stage the next month for a three-night celebration of 60 years of music at Golden Gate Park. His family added: “Bobby will forever be a guiding force whose unique artistry reshaped American music. His work did more than fill rooms with music; it was warm sunlight that filled the soul, building a community, a language and a feeling of family that generations of fans carry with them. Every chord he played, every word he sang was an integral part of the stories he wove. There was an invitation: to feel, to question, to wander, and to belong.”  As the leader of such bands as Dead and Company, Phil Lesh and Friends, Further, Rat …