ci: add shellcheck linting for shell scripts

Add shellcheck to CI pipeline to ensure shell scripts follow best practices
and catch common errors. Fix existing shellcheck warnings in test scripts
to pass the new linting requirements.

nix/devShell.nix

@@ -61,6 +61,7 @@
   pinact,
   hyperfine,
   typos,
+  shellcheck,
   uv,
   wayland,
   wayland-scanner,
@@ -101,6 +102,7 @@ in
         alejandra
         pinact
         typos
+        shellcheck
 
         # Testing
         parallel