Cloudflare Containers, rebuilt to scale agent sandboxes

This post details the rearchitecture of Cloudflare Containers to support agent sandboxes. Key technical contributions include the introduction of a `durable_object` scheduling policy that allows runtime selection of sandbox images and instance types, a redesigned runtime that reduces median startup time from 4.049s to 648ms, and the public beta of filesystem snapshots. The changes shift control over sandbox configuration from deploy-time to runtime code within the Durable Object, enabling more flexible rollouts and faster initialization.