I've been playing with nested sessions and I see that, of course, the parent ...

I've been playing with nested sessions and I see that, of course, the parent can see the child and viceversa, but noticed that really any session can see any other open session. So two things
1. Sometimes I may have ton of sessions and I don't want a session "tree" from being able to see another stree (or it might get confusing for that agent). Similarly, it would be great to control if sessions can see their siblings or just parent/child. A setting to control how this discovery behavior works would be awesome
2. Sometimes I've wanted a session tree to be reparented under another. Right now this forks in practice because any session can see any other so I can just tell "root A" to take charge of "root B". But this change is not really reflected in the UI. It would be cool if we could move a "tree" from one place to another

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| Field | Value |
| --- | --- |
| App version | 1.1.0 |
| OS | Windows 10.0.26200 |
| Theme | Fox |
| Path | /chat |
| Tenure | Week 3 |

从 /chat 路径开始，复现 issue 中描述的嵌套会话行为，包括不相关会话之间的可见性，以及一个会话接管另一个会话后当前 UI 的状态。完成的标准是：可以控制不同会话树的发现行为，以及父子会话或兄弟会话之间的可见性，并且移动会话树会反映在 UI 中。

由索引模型根据 Issue 内容生成。