feat: 线程等待与状态读取 8 项（wait/read/message/handoff/worktree/pr-checks/automation/sleep）

对照 Codex app：wait_threads 首变+afterCursor+8上限、
read_thread 详情摘要、send_message_to_thread 复活失败任务、
handoff 登记与上报轮询、worktree 派生状态、PR checks JSON+
check-runs 日志与注释、automation GET/PATCH、clock.sleep
（API+host）。诚实边界见 docs/thread-wait-ops.md：无
worktree 阶段流、无 automation 变化静默、无独立测试结果
记录、handoff 服务端不执行迁移、wait 仅数据库级变化检测。

三套件：cloud 940 OK（4 跳过）、host 91 OK、deploy 29 OK

cloud/migrations.py

@@ -104,6 +104,7 @@ UNMIGRATED_TABLES = (
     "github_fix_review_dedup",
     "external_identities",
     "pr_jobs",
+    "task_handoffs",
     "rate_limit_hits",
     "user_gateway_keys",
     "webhook_deliveries",