Copilot CLI 1.0.83 stable lets a custom agent list several models and fall through them in order

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Shipped 2026-09-04, custom agents can name several models in `model`, tried in order until one is available to you, and `model-policy: required` keeps model changes on that list. This is the first mainstream agent CLI to make heterogeneous model routing declarative rather than a wrapper script, and the policy flag is the piece that makes it usable in a team, since it stops a teammate silently switching a cost-sensitive agent onto a frontier model.