gui: stop rebuilding session tree on selection changes

The dominant 9P spam came from buildSessionTree() re-listing
session/<sid>/agent on every activeSessionIdChanged and
activeAgentIdChanged signal. Tree rebuilds are now limited to
sessionsChanged and startup refresh; selection changes only
refresh the right pane transcript.

gui/main.qml

@@ -218,7 +218,6 @@ ApplicationWindow {
                 }
             }
             scrollTimer.start()
-            buildSessionTree()
         }
         function onActiveAgentIdChanged() {
             chatModel.reset()
@@ -230,7 +229,6 @@ ApplicationWindow {
                 }
             }
             scrollTimer.start()
-            buildSessionTree()
         }
         function onStreamingDone() {
             chatModel.appendData("\n[[[info]]]\nsession ended\n[[[end]]]\n")