Workflow SDK now runs natively in Nitro v3

Workflow SDK beta now runs natively in Nitro v3 with steps executing in the same runtime as the app. useStorage and other Nitro APIs work directly inside "use step" functions. workflows/sync-user.ts demonstrates reading from Nitro storage in a step. Workflow UI serves at /_workflow and is bundled with app code. Unused code is tree-shaken for smaller, faster builds.