gitconfig: use absolute include path for .gitconfig.user

The previous `[include] path = .gitconfig.user` was resolved relative to
the config file being read, which — because `~/.gitconfig` is a symlink
to `~/.yadr/git/gitconfig` — meant git looked for the user-identity
override at `~/.yadr/git/.gitconfig.user` instead of the conventional
`~/.gitconfig.user`. Switch to an absolute path so the include picks up
the homedir file regardless of symlink layout.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

git/gitconfig

@@ -126,4 +126,4 @@
   # http://gitfu.wordpress.com/2008/04/20/git-rerere-rereremember-what-you-did-last-time/
   enabled = true
 [include]
-  path = .gitconfig.user
+  path = ~/.gitconfig.user