refactor(cli): unify thread storage, remove history prefix

- store.ts: all threads in @uwf/thread/* with status tag
- Remove HISTORY_VAR_PREFIX, ThreadHistoryLine, deleteThread
- Add loadActiveThreads, loadHistoryThreads, completeThread
- Add migrateHistoryVarsToThreadVars migration
- thread.ts: replace deleteThread+addHistoryEntry with completeThread
- shared.ts: remove findHistoryEntry fallback
- Update all tests for unified storage model

422 tests pass.

Part of #39, closes #41, closes #42

packages/util-agent/src/storage.ts

@@ -88,8 +88,10 @@ export async function getActiveThreadEntry(
   }
   return {
     head: v.value as CasRef,
+    status: (v.tags.status ?? "idle") as ThreadIndexEntry["status"],
     suspendedRole: v.tags.suspendedRole ?? null,
     suspendMessage: v.tags.suspendMessage ?? null,
+    completedAt: v.tags.completedAt !== undefined ? Number(v.tags.completedAt) : null,
   };
 }