Agent UI: opening a folder should focus it in file tree, not open a pane

[hachej]

Bug

When the agent UI handles an “open folder” action, it currently opens a pane for the folder.

Expected behavior

Opening a folder should only focus/select/reveal that folder in the file tree. No editor/pane should be opened.

Notes

Likely route/command handling should distinguish file opens from folder reveals. Folder targets should dispatch a file-tree focus/reveal action only.