fix: reload CAS store after agent spawn + share schemas via uwf-protocol

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

scripts/mock-agent.ts

@@ -0,0 +1,12 @@
+#!/usr/bin/env bun
+// Mock agent for smoke testing
+import { createAgent } from "../packages/uwf-agent-kit/src/index.js";
+
+const agent = createAgent({
+  name: "mock",
+  run: async (ctx) => {
+    return `Mock output for role ${ctx.role}: task was "${ctx.prompt}"`;
+  },
+});
+
+await agent();