Merge branch 'main' of https://gitea.federated.computer/Federated_Computer/coredesktop

Brings in the shellcheck lint and bats tests. Both sides fixed the same
shellcheck warnings; this keeps the remote's versions of 20-aur.sh,
80-gnome-extensions.sh and .shellcheckrc, which silence false positives
per line rather than for the whole repo.

The Gitea workflow now runs make lint, so it checks the same scripts as
the Makefile, and the README points contributors at make check. The test
helper's REPO, used only by the .bats files, is marked for shellcheck,
and the update test expects the pull to be fast-forward only.

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

system/backup

@@ -27,7 +27,7 @@ source "$COREDESKTOP/system/backup-env.sh" || exit 1
 [ -n "$RESTIC_PASSWORD_COMMAND" ] || fail "The backup's passphrase isn't in your keyring. Run: configure backup"
 
 # Run at login, perhaps before the network is up; give it a few minutes
-for try in {1..10}; do
+for _ in {1..10}; do
   curl -sf --max-time 10 -o /dev/null "${BACKUP_WEBDAV%%/remote.php/*}/status.php" && break
   sleep 30
 done