Workflow SDK adds idempotent starts

Version 5.4.0 of Vercel's Workflow SDK lets an application start a workflow run at most once per key, even when the same request arrives twice. The SDK runs workflows built from steps that the framework can retry. It also has hooks: channels identified by a token that let outside code send data into a running workflow with `resumeHook()`. According to the changelog, the previous approach was to look a hook up with `getHookByToken()` and then call `start()`, and the gap between those two calls left a documented race in which the same run could be started twice.

In the new version, `start()` accepts an `experimental_hook` option with a token and admits the run and claims that token in one atomic step. The token comes from the application's own data, such as an order or invoice ID. Exactly one run owns it; a duplicate `start()` throws `HookConflictError` carrying the owner's `conflictingRunId`, and the duplicate never becomes a run. Because the hook exists from the moment the run is admitted, the duplicate request's data can be forwarded to the owning run with `resumeHook()`, and payloads sent before the workflow starts listening are kept and delivered in order.

The guarantee lasts only while the token is held. The idempotency documentation says the token is released when the run ends unless `experimental_minRetention` is set, so without it a late duplicate starts a new run; there is no default retention. Disposing the start hook inside the workflow also releases the token at once, even while the run continues.