The agent subprocess writes StepNode to CAS on disk, but the parent
process had an in-memory cache from createFsStore init. Fix: re-create
store after agent spawn to pick up new nodes.
Also centralized JSON Schemas in uwf-protocol so cli-uwf and agent-kit
produce identical type hashes.
E2E smoke test passing: workflow put → thread start → 3x step → done
Refs #309