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/cli/src/__tests__/thread-suspend-step.test.ts

@@ -160,8 +160,10 @@ describe("suspend step CAS chain and threads.yaml metadata", () => {
       const threadEntry = getThread(uwf.varStore, threadId);
       expect(threadEntry).toEqual({
         head: stepHash,
+        status: "suspended",
         suspendedRole: "worker",
         suspendMessage: "Please clarify: Which API?",
+        completedAt: null,
       });
 
       const showResult = await cmdThreadShow(tmpDir, threadId);