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/commands/shared.ts

@@ -6,7 +6,7 @@ import type {
   StepNodePayload,
   ThreadId,
 } from "@united-workforce/protocol";
-import { createUwfStore, findHistoryEntry, getThread, type UwfStore } from "../store.js";
+import { createUwfStore, getThread, type UwfStore } from "../store.js";
 
 type ChainState = {
   startHash: CasRef;
@@ -207,10 +207,6 @@ async function resolveHeadHash(storageRoot: string, threadId: ThreadId): Promise
   if (entry !== null) {
     return entry.head;
   }
-  const hist = findHistoryEntry(uwf.varStore, threadId);
-  if (hist !== null) {
-    return hist.head;
-  }
   fail(`thread not found: ${threadId}`);
 }