[GIT] Make core.excludesfile path relative for portability

Was hardcoded to /Users/ising/dotfiles; use ~/dotfiles so the base config
works on non-macOS hosts too.

(Separately, a local uncommitted 'credential.helper = store' was removed from
the live config so git uses the macOS keychain instead of writing
~/.git-credentials in plaintext. That line was never committed, so there is no
diff for it here.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

gitconfig/gitconfig

@@ -25,7 +25,7 @@ email = github@murgi.de
 [includeIf "hasconfig:remote.*.url:https://git-int.sit.fraunhofer.de/**"]
     path = ~/dotfiles/gitconfig/sit.gitconfig
 [core]
-	excludesfile = /Users/ising/dotfiles/.gitignore_global
+	excludesfile = ~/dotfiles/.gitignore_global
 [submodule]
 	recurse = true
 [filter "lfs"]