Rearchitecting the Workflows control plane for the agentic era

The Workflows control plane was rearchitected from V1 to V2 to support increased scale driven by agentic workloads. V1 used a single account-level Durable Object (DO) as a bottleneck. V2 introduces SousChef (manages subsets of instances per workflow) and Gatekeeper (distributes concurrency slots) to horizontally scale the system. The source of truth for instance existence is now its Engine DO, and instance lifecycle mechanisms are horizontally scalable and distributed. The new Account singleton stores minimal metadata and has an invariant maximum concurrent requests. The migration process involved seamlessly transitioning customers and instances to the new V2 architecture without downtime.