fix: agent createAgentStore uses wrong CAS path (~/.uwf/cas instead of ~/.ocas)

createAgentStore was calling getCasDir(storageRoot) which resolves to
~/.uwf/cas/, but since Phase 3 all CAS data lives in ~/.ocas/.
getActiveThreadEntry already used getGlobalCasDir() correctly, causing
a split where thread lookup succeeded but CAS node reads failed.

Found during e2e walkthrough after Phase 0-5 migration.

packages/workflow-util-agent/src/storage.ts

@@ -108,7 +108,7 @@ export type AgentStore = {
 };
 
 export async function createAgentStore(storageRoot: string): Promise<AgentStore> {
-  const store = createFsStore(getCasDir(storageRoot));
+  const store = createFsStore(getGlobalCasDir());
   const schemas = await registerAgentSchemas(store);
   return { storageRoot, store, schemas };
 }