refactor: WorkflowFn input → ThreadInput, remove threadId from bundle contract

- WorkflowFn first param is now ThreadInput { prompt, steps }
- threadId removed from WorkflowFnOptions and ThreadContext (engine-only)
- createRoleModerator seeds context from input.steps (fork/resume ready)
- New test: pre-filled steps skip already-completed roles

Closes #6
小橘 <xiaoju@shazhou.work>

packages/workflow/src/index.ts

@@ -47,6 +47,7 @@ export {
   START,
   type StartStep,
   type ThreadContext,
+  type ThreadInput,
   type WorkflowDefinition,
   type WorkflowFn,
   type WorkflowFnOptions,