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>

README.md

@@ -326,7 +326,7 @@ This whole "project" is licensed under the MIT License. Want to help make CoreDe
 
 Found a bug or have an idea? [Open an issue](https://gitea.federated.computer/Federated_Computer/coredesktop/issues).
 
-Every push is checked with [shellcheck](https://www.shellcheck.net) (settings in .shellcheckrc). Run `shellcheck` on the scripts you change before sending a PR.
+Every push is checked with [shellcheck](https://www.shellcheck.net) (settings in .shellcheckrc). Before sending a PR, run `make check`: it runs shellcheck (`make lint`) and the [bats](https://github.com/bats-core/bats-core) tests in tests/ (`make test`).
 
 Changes to install.sh only reach new installs. If existing machines need the change too, also add a migration: a script named `migrations/$(date +%s).sh` that `update` runs once on each machine. It may also run on fresh installs, so make it safe to repeat.