feat: 扩充 shell 命令白名单(含 containerd/ctr/nerdctl)，解除 AI Agent 轮数上限

- 新增 ~50 个 Linux 基础命令(du/top/iostat/sar/curl/tcpdump 等)
- 新增 containerd 系列: ctr (containers/images/tasks/snapshots),
  nerdctl (ps/logs/inspect/stats 等)
- ip 命令补充 maddr/monitor/tunnel/tuntap 只读操作
- _run_agent_loop 改为 while True 循环，max_steps=0 表示不限轮数
- main.py 传入 max_steps=0，AI Agent 持续诊断直到问题解决
- 更新 Agent system prompt 第7条：明确无轮数上限

backend/main.py

@@ -327,6 +327,7 @@ async def agent_diagnose_stream(question: str = "", execution_mode: str = "ai"):
             async for event in run_diagnostic_agent(
                 _last_report,
                 question=question,
+                max_steps=0,
                 execution_mode=execution_mode,
             ):
                 history_store.append_event(run_id, event.get("type", "event"), {k: v for k, v in event.items() if k != "type"})