feat(editor): introduce split pane manager plugin and refactor source view system

* address TODO item for split_pane_manager plugin and bound keys
* add split_pane_manager plugin with create/close split view commands
* move sibling focus/move commands from core into plugin system
* remove legacy toggle_source_view plugin
* redesign EditorsContainer to use simpler Box-based layout
* refactor source view API to return (scrolled_window, source_view)
* add source view lifecycle events (create/remove/removed)
* rename GetNewCommandSystemEvent → CreateCommandSystemEvent
* update CommandsController to support command system creation and cleanup
* ensure command systems are removed with their source views
* improve focus border handling across sibling chains
* update telescope integration for new source view structure
* clean up container imports and initialization logic
* remove old keybindings and align with new split pane workflow

plugins/code/ui/telescope/plugin.py

@@ -52,8 +52,8 @@ class Plugin(PluginCode):
         )
         self.emit_to("source_views", event)
 
-        source_view = event.response
-        telescope.set_source_view(source_view)
+        scrolled_win, source_view = event.response
+        telescope.set_source_view(scrolled_win, source_view)
 
         event = Event_Factory.create_event(
             "register_completer",

plugins/code/ui/telescope/telescope.py

@@ -99,13 +99,9 @@ class Telescope(Gtk.Dialog):
     def unmap_parent_resize_event(self, parent):
         parent.disconnect(self.size_allocate_id)
 
-    def set_source_view(self, source_view):
-        scrolled_win     = Gtk.ScrolledWindow()
+    def set_source_view(self, scrolled_win, source_view):
         self.source_view = source_view
-
-        scrolled_win.add(self.source_view)
         self.main_box.pack_end(scrolled_win, True, True, 0)
-
         scrolled_win.show_all()
 
     def _handle_destroy(self, widget):