Who believed in you first? Silicon Valley runs on that question. a16z's Erik Torenberg, David Booth, Josh Elman, and Olivia Moore on Cosign: Cosign is a curated professional network for the startup community. Profiles are built from endorsements, fundraises, and talent moves, with a private layer to signal who you'd fund or hire. Erik ran a small version of this seven years ago, emailing a few hundred people and asking each for one person to watch. One of the answers was Russell Kaplan, then…