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

user_config/usr/share/newton/code-key-bindings.json

@@ -44,18 +44,6 @@
         },
         "toggle_plugins_ui": {
             "released": "<Control><Shift>p"
-        },
-        "focus_left_sibling": {
-            "released": "<Control>Page_Up"
-        },
-        "focus_right_sibling": {
-            "released": "<Control>Page_Down"
-        },
-        "move_to_left_sibling": {
-            "released": "<Control><Shift>Up"
-        },
-        "move_to_right_sibling": {
-            "released": "<Control><Shift>Down"
         }
     }
 }