Fix test: use execute instead of succeed for pkill to handle exit code 137

lab/test-resource-guard.nix

@@ -221,7 +221,8 @@ in
         print("  ✓ Agent terminated with SIGKILL")
     else:
         print("  ⚠ Agent still running, will kill all instances")
-        client.succeed("pkill -9 -f 'lightscale-client.*agent' || true")
+        # Use execute instead of succeed to ignore exit code
+        client.execute("pkill -9 -f 'lightscale-client.*guard' || true")
         time.sleep(2)
 
     # Note: SIGKILL cleanup happens via Drop trait, but timing may vary