Install VSCodium only, and remove VS Code

New machines get VSCodium alone. A migration uninstalls the VS Code
Flatpak once on existing machines; its settings stay in ~/.var/app. The
README says how to install VS Code for extensions only Microsoft's
marketplace has, such as Remote-SSH; update then leaves it alone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/1793457205.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+# CoreDesktop installs VSCodium, not VS Code. Remove VS Code once; its settings
+# stay in ~/.var/app/com.visualstudio.code. For extensions only Microsoft's
+# marketplace has, such as Remote-SSH, install it again yourself and update
+# leaves it alone: flatpak install flathub com.visualstudio.code
+if flatpak info --system com.visualstudio.code &> /dev/null; then
+  echo "Removing VS Code (VSCodium stays)..."
+  sudo flatpak uninstall --system --noninteractive com.visualstudio.code || exit 1
+fi
+exit 0