Start a local multi-node Temps cluster (Docker-in-Docker)

Wraps tools/dev-cluster/ — a docker-compose harness that runs a real Temps control plane plus 3 worker nodes, each a privileged DinD container, with the actual overlay network (VXLAN, compute_cidr allocation, cluster DNS) wired up exactly as it would be on real hardware. Use this whenever a fix or feature needs more than one node to prove anything — single-node start-temps cannot exercise cross-node scheduling, the overlay, or cluster DNS at all.

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