Sync local state with remote after untracked local evolution

Local site-ai/ had diverged significantly from the last commit pushed
to Gitea (new remote-env-setup/remove targets, SITE_CONFIG_DIR/MAIN_REPO
split, credential cleanup in server-install.sh, migration docs) without
ever being committed. This catches the repo up to what's actually on disk.

Also untracks the legacy user/ subtree left over from before content was
split into its own standalone repo (natascha-rieter.nl-user) — user/ is
gitignored here and stays a separate git repo, unaffected by this commit.

plugins.txt

@@ -13,3 +13,4 @@ automagic-images
 admin-media-move
 admin-media-replace
 admin-media-actions
+git-sync