Add workflow.start(), workflow.advance(), workflow.cancel(), and
workflow.submit_signoff() to the Starlark workflow module. Extract
WorkflowEngine interface in sandbox package to break the circular
import between workflow and sandbox packages.
- WorkflowEngine interface (Start/Advance/Cancel/SubmitSignoff)
- instanceToDict/signoffToDict/dictToJSON shared helpers
- Runner.SetWorkflowEngine setter + main.go wiring
- 6 new unit tests with mock engine
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
