A West Marin sky swiped with long streaks of blue looked down as John Francis walked the main road of Point Reyes Station, a banjo tucked into a green travel bag slung from his shoulder. The scene was a reenactment of sorts — shades of a day in the early ’70s when Francis decided to forego riding in cars and instead walk in response to the 1971 oil spill in the San Francisco Bay after two tankers collided. “I decided that I was responsible for s…