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>

.shellcheckrc

@@ -0,0 +1,7 @@
+# Hooks, configure items and config files are sourced from paths only known at
+# run time
+disable=SC1090
+
+# mkdir -p -m only sets the mode of the last directory; every use here makes
+# one directory in a parent that already exists
+disable=SC2174