fix(shell): add explicit return statements to functions

Add return 0 as the last statement in ~46 shell functions across
17 files that previously relied on implicit return codes
(SonarCloud rule shelldre:S7682).

add-submodules.sh

@@ -48,6 +48,7 @@ _log() {
   else
     echo "  [ok] $1"
   fi
+  return 0
 }
 
 remove_old_submodule() {