Desktop: Sidebar subsessions support

[dbpolito]

Adding support for showing subsessions on sidebar, collapsing similar to project:

Right now i kept a similar behavior from project, clicking the row expand / collapse and navigate to it.

[adamdotdevin]

So, I used to show child sessions in the sidebar, but tbh, I don't think they belong there. I would accept a PR for making the Task tool description a link to the child session, though. That was my plan, just hadn't gotten to it.

[dbpolito]

@adamdotdevin hmm, i think the general ux of navigating between subsessions aren't great on both desktop and tui...

I opened #4564 a while back, there is also #4865 that brings them to the sidebar...

Let's think about some other options:

1. Separate the expand / collapse behavior so you really need to click on the icon to open / view sub sessions, that way people can easily ignore sub sessions?
2. Maybe add a button on the new status bar? x subsessions? you click, we open a modal with a list of sessions? similar to my PR on TUI? that would be somehow consistent, we could even have the same /subagent-sessions command to open the dialog (or maybe just the command is enough)

I think clicking the task tool description is fine but then you would navigate to it, how you would return to the parent session? You would need to find it back on the sidebar? Feel a bit weird