feat(desktop): Agent Graph 节点 TPS 与输出 preview

## 需求

在 Desktop Agent Graph 面板中，为每个 graph node / operator 增加：
- 实时 TPS（token/s）
- 输出 preview（运行中流式摘要 / 完成后结果摘要）
- 保留“打开子任务”进入完整 child Session

## OpenSpec

已规划 ，包含 proposal / specs / design / tasks。

## 范围

- 不改变 Graph 调度/执行/恢复语义
- 优先 Desktop GUI；CLI/TUI 可后续跟随

Start with the referenced OpenSpec proposal, specs, design, and tasks, then locate the Desktop Agent Graph panel and the child Session entry point. Done means each node or operator shows live token/s and streaming or completed output preview while graph scheduling, execution, recovery, and child-session navigation remain unchanged.

Written by the indexing model from the issue text.