Challenge-level bounded concurrency must preserve per-challenge agent ordering and isolate mutable git worktrees

A benchmark orchestrator added bounded parallel execution by scheduling each agent/challenge pair independently. That did not match a spec requiring challenge-level parallelism: agents for the same challenge must run sequentially, while separate challenges run concurrently. It also risked repository checkout races because concurrent challenge jobs could share a single cloned repo while each job checked out a different affected version.