Agents that start from a repository start their sessions

An agent whose definition sets from: {"repo": "owner/name"} starts each session from that repository's CI/CD warm snapshot. Previously every session of such an agent ended immediately with sandbox_refused (403 forbidden, "a create naming another snapshot"), because the session's key was allowed to create its sandbox only from a snapshot the definition names by id. A session's key may now create its sandbox from any ready warm snapshot of the definition's repository, and from no other snapshot.