“I have spent my life/Seeking all that’s still unsung/Bent my ear to hear the tune,” sang Phil Lesh last night, harmonizing with colleagues new and old, on “Attics of My Life,” the final song of a fraught, moving, ultimately magnificent five-night, two-state Fare The Well concert series — billed as the final shows that the surviving members of the Grateful Dead will ever perform together. The final concert was also the run’s strongest, showcasing a new band hitting its stride precisely as it was set to retire.