refactor: remove split_pane_manager plugin and harden view handling

- Delete deprecated split_pane_manager command plugin and all related commands
- Introduce new split_pane command structure (untracked replacement)
- Add guard in code_minimap to handle missing active view
- Prevent language detection for non-file buffers in command_helpers

This cleans up legacy split pane logic and improves stability for edge cases.

plugins/code/commands/split_pane/focus_left_sibling.py

@@ -0,0 +1,22 @@
+# Python imports
+
+# Lib imports
+import gi
+
+gi.require_version('GtkSource', '4')
+
+from gi.repository import GtkSource
+
+
+# Application imports
+
+
+
+def execute(
+    view: GtkSource.View,
+    *args,
+    **kwargs
+):
+    logger.debug("Command: Focus Left Sibling")
+    if not view.sibling_left: return
+    view.sibling_left.grab_focus()