fix(git): back up a stray ~/.gitconfig after writeBoundary

Move it aside with HM's `run mv` (dry-run aware) instead of an
unconditional rm before checkLinkTargets.

Assisted-by: Claude Code (claude-opus-5-5)

home/base/core/git.nix

@@ -1,5 +1,4 @@
 {
-  config,
   lib,
   pkgs,
   myvars,
@@ -10,8 +9,10 @@
   # to make git use this config file, `~/.gitconfig` should not exist!
   #
   #    https://git-scm.com/docs/git-config#Documentation/git-config.txt---global
-  home.activation.removeExistingGitconfig = lib.hm.dag.entryBefore [ "checkLinkTargets" ] ''
-    rm -f ${config.home.homeDirectory}/.gitconfig
+  home.activation.backupExistingGitconfig = lib.hm.dag.entryAfter [ "writeBoundary" ] ''
+    if [[ -e ~/.gitconfig ]]; then
+      run mv $VERBOSE_ARG ~/.gitconfig ~/.gitconfig.bak
+    fi
   '';
 
   # GitHub CLI tool