This adds the onboarding help topic: atgc agent onboarding takes an agent from the first login to the first open pull. It gives the order nothing else states (log in, create a key, clone over SSH), then the sidecar-or-fork branch, doctor local, and the exit-2 and exit-4 failures at the first push.
The order silently breaks when ignored: repo clone before key create gives an HTTPS remote that cannot push (src/cmd/repo/checkout.rs:636), auth login blocks for up to 300 s off a terminal, a hand push to a git@ URL can wait an hour (src/clients/tangled/resolve.rs:180), and the push-failure hint sent a collaborator to fork (dogfood r4 #2).
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