feat(gui): track VSCode diff process, close it when patch modal closes

- Store _vscode_diff_process after launching external editor
- Add _close_vscode_diff() helper to terminate the process
- Call _close_vscode_diff() when Apply Patch or Reject is clicked

src/gui_2.py

@@ -2201,8 +2201,10 @@ class App:
    imgui.same_line()
    if imgui.button("Apply Patch"):
     self._apply_pending_patch()
+    self._close_vscode_diff()
    imgui.same_line()
    if imgui.button("Reject"):
+    self._close_vscode_diff()
     self._show_patch_modal = False
     self._pending_patch_text = None
     self._pending_patch_files = []
@@ -2252,9 +2254,18 @@ class App:
     self._patch_error_message = "Failed to launch external editor"
    else:
     self._patch_error_message = None
+    self._vscode_diff_process = result
   except Exception as e:
    self._patch_error_message = str(e)
 
+ def _close_vscode_diff(self) -> None:
+  if hasattr(self, '_vscode_diff_process') and self._vscode_diff_process:
+   try:
+    self._vscode_diff_process.terminate()
+   except Exception:
+    pass
+   self._vscode_diff_process = None
+
  def request_patch_from_tier4(self, error: str, file_context: str) -> None:
   try:
    from src import ai_client