byre never signs with a key on the machine it runs on: its own end-of-turn and snapshot commits there are always unsigned. When a project checkout (or the server's global config) has `commit.gpgsign` true, the squash commit of a Merge or `merge_to_main` is signed on the merging user's most recently active connected Mac, so a key that never leaves that Mac (a Secure Enclave key behind Touch ID, say) works. The merge fails when that user has no app connected.
`<checkout>` is the project's main checkout. It sets `commit.gpgsign` true for the checkout, prints it, and fails unless it reads true. No restart is needed; the next merge reads the config.
