fix: guard empty array iteration for bash 3.2 compatibility

The _load_repo_dotenv_preserving_env() function iterates over
${preserved[@]} with set -euo pipefail. On bash 3.2 (macOS default),
an empty array triggers 'unbound variable' under set -u, crashing
ctl.sh start. Bash 4+ handles this fine, but macOS ships 3.2.

Wraps the for loop in a length check: [[ ${#preserved[@]} -gt 0 ]]

ctl.sh

@@ -51,9 +51,11 @@ _load_repo_dotenv_preserving_env() {
   set +a
 
   local assignment
-  for assignment in "${preserved[@]}"; do
-    export "${assignment}"
-  done
+  if [[ ${#preserved[@]} -gt 0 ]]; then
+    for assignment in "${preserved[@]}"; do
+      export "${assignment}"
+    done
+  fi
 }
 
 _find_python() {