Attach a git repository and return the public key to register

Attach a git repository (phase 1): validate the config, generate the SSH keypair, persist it, and notify the runners. Returns the PUBLIC key plus a "key_registration" object saying where it must be registered — git is not enabled until it is and /status flips to "connected". key_registration.target is "repository_deploy_key" for GitHub/GitLab/Bitbucket, or "account_ssh_key" for Azure DevOps, which has no per-repository deploy keys: there the key binds to a user identity and inherits everything that account can reach, so key_registration.warning is populated and must be shown. Admin only.