Add shellcheck lint and bats tests

make lint runs shellcheck on every tracked bash script, make test runs
the bats suite in tests/, and make check runs both. The tests cover the
update orchestrator (hook isolation, failure reporting, pull and
free-space handling) and utils.sh (migrations, package install and
removal, services, initramfs build check), using stub commands so they
never touch the real system.

Fixes from the first lint run: 20-aur.sh stops if a cd fails rather
than building or deleting in the wrong directory, and
80-gnome-extensions.sh refuses to rm -rf with an empty path. The rest
are false positives, silenced in .shellcheckrc or per line.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

configure

@@ -96,7 +96,7 @@ load_items() {
 show_items() {
   local i mark
   for i in "${!ITEMS[@]}"; do
-    if [ "${STATES[$i]}" = done ]; then
+    if [ "${STATES[$i]}" = "done" ]; then
       mark="\e[32mdone\e[0m "
     else
       mark="\e[33mto do\e[0m"