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/manifest.json

@@ -0,0 +1,7 @@
+{
+    "name": "Split Pane",
+    "author": "ITDominator",
+    "version": "0.0.1",
+    "support": "",
+    "requests": {}
+}