feat(git): deploy user-scope ~/.gitconfig from the repo template

Git never expands $VARS in its config, so gitconfig carries @USER@ and
@EMAIL@ placeholders that install.sh fills from the deployed bashrc's
USER/EMAIL exports. A differing ~/.gitconfig is kept as
~/.gitconfig.backup-<date>; an identical one is left alone. A repo's
.git/config still overrides it. excludesfile uses ~ (git expands it,
not $HOME).

The rc files now override $USER, so the installer takes the login name
from id -un for dscl and the code-server unit.

gitconfig

@@ -0,0 +1,31 @@
+# Template for the user-scope ~/.gitconfig, rendered by install.sh.
+# Git never expands $VARS: @USER@ and @EMAIL@ are replaced at install
+# time with the USER and EMAIL exported by the deployed bashrc.
+# A repo .git/config still overrides these values for that repo.
+[user]
+    name = @USER@
+    email = @EMAIL@
+[push]
+    default = current
+[color]
+    ui = auto
+[pull]
+    rebase = true
+[core]
+    editor = vim
+    pager = delta
+    excludesfile = ~/.gitignore
+[advice]
+    detachedHead = false
+[merge]
+    tool = vimdiff
+    conflictStyle = zdiff3
+[pager]
+    branch = false
+[url "git@salsa.debian.org:installer-team/"]
+    pushInsteadOf = https://salsa.debian.org/installer-team/
+[interactive]
+    diffFilter = delta --color-only
+[delta]
+    navigate = true  # use n and N to move between diff sections
+    dark = true      # or light = true, or omit for auto-detection