Joan Baez, Taj Mahal and Sammy Hagar perform at Acoustic-4-A-Cure at The Fillmore in San Francisco on May 13, 2023. Steve Carlson/STAFF. SAN FRANCISCO — An unexpected appearance by Joan Baez, impressive blues chops of Taj Mahal and punchy one-liners by Chris Isaak highlighted the Acoustic-4-a-Cure concert at the…