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>

claude.sh

@@ -15,10 +15,10 @@ fi
 # The native installer places the binary in ~/.local/bin
 LINE='export PATH="$HOME/.local/bin:$PATH"'
 if grep -qF "$LINE" ~/.bashrc; then
-  echo "~/.local/bin already on PATH in .bashrc"
+  echo "\~/.local/bin already on PATH in .bashrc"
 else
   echo "$LINE" >> ~/.bashrc
-  echo "~/.local/bin added to PATH in .bashrc"
+  echo "\~/.local/bin added to PATH in .bashrc"
 fi
 
 # Desktop notifications via notify-send (libnotify)