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.test.ts

@@ -11,7 +11,7 @@ import {
   THREAD_READ_DEFAULT_QUOTA,
 } from "../commands/thread.js";
 import type { UwfStore } from "../store.js";
-import { addHistoryEntry, createUwfStore } from "../store.js";
+import { completeThread, createUwfStore, setThread } from "../store.js";
 import { seedThreads } from "./thread-test-helpers.js";
 
 // ── schemas used in tests ────────────────────────────────────────────────────
@@ -745,13 +745,14 @@ describe("cmdStepList with completed threads", () => {
     const threadId = "01JTEST0000000000000000A2" as ThreadId;
     // Thread is NOT in active index (simulating completed thread)
     // But it IS in history variable store
-    addHistoryEntry(uwf.varStore, {
-      thread: threadId,
-      workflow: workflowHash,
+    setThread(uwf.varStore, threadId, {
       head: step2Hash,
-      completedAt: Date.now(),
-      reason: null,
+      status: "idle",
+      suspendedRole: null,
+      suspendMessage: null,
+      completedAt: null,
     });
+    completeThread(uwf.varStore, threadId, "completed");
 
     const result = await cmdStepList(tmpDir, threadId);
 
@@ -872,14 +873,15 @@ describe("cmdStepShow with completed threads", () => {
 
     const threadId = "01JTEST0000000000000000B2" as ThreadId;
     // Thread is NOT in active index
-    // But it IS in history variable store
-    addHistoryEntry(uwf.varStore, {
-      thread: threadId,
-      workflow: workflowHash,
+    // But it IS in the unified store with completed status
+    setThread(uwf.varStore, threadId, {
       head: stepHash,
-      completedAt: Date.now(),
-      reason: null,
+      status: "idle",
+      suspendedRole: null,
+      suspendMessage: null,
+      completedAt: null,
     });
+    completeThread(uwf.varStore, threadId, "completed");
 
     const result = await cmdStepShow(tmpDir, stepHash);
 
@@ -934,15 +936,15 @@ describe("cmdThreadRead with completed threads", () => {
     });
 
     const threadId = "01JTEST0000000000000000C1" as ThreadId;
-    // Thread is NOT in active index
-    // But it IS in history variable store
-    addHistoryEntry(uwf.varStore, {
-      thread: threadId,
-      workflow: workflowHash,
+    // Thread is in store with completed status
+    setThread(uwf.varStore, threadId, {
       head: stepHash,
-      completedAt: Date.now(),
-      reason: null,
+      status: "idle",
+      suspendedRole: null,
+      suspendMessage: null,
+      completedAt: null,
     });
+    completeThread(uwf.varStore, threadId, "completed");
 
     const markdown = await cmdThreadRead(tmpDir, threadId, THREAD_READ_DEFAULT_QUOTA, null, false);
 
@@ -998,13 +1000,14 @@ describe("cmdThreadRead with completed threads", () => {
     });
 
     const threadId = "01JTEST0000000000000000C2" as ThreadId;
-    addHistoryEntry(uwf.varStore, {
-      thread: threadId,
-      workflow: workflowHash,
+    setThread(uwf.varStore, threadId, {
       head: step3Hash,
-      completedAt: Date.now(),
-      reason: null,
+      status: "idle",
+      suspendedRole: null,
+      suspendMessage: null,
+      completedAt: null,
     });
+    completeThread(uwf.varStore, threadId, "completed");
 
     const markdown = await cmdThreadRead(
       tmpDir,