Say what the sweep helper is, when run on its own

A suite runner walking the tests directory executes it with no arguments
and gets an IndexError, which reads as a failing test rather than a
helper being used wrongly.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

tests/quickshell/settings_write_sweep.py

@@ -178,6 +178,14 @@ def ipc(config_home: str, harness: str, *arguments: str) -> subprocess.Completed
 
 
 def main() -> int:
+    # Driven by settings-write-sweep-contract.sh, which starts the harness this
+    # needs and restores the desktop afterwards. Run bare -- by a suite runner
+    # walking the directory, say -- it says so instead of failing on argv.
+    if len(sys.argv) < 3:
+        print("settings_write_sweep is driven by tests/quickshell/"
+              "settings-write-sweep-contract.sh; run that instead.")
+        return 0
+
     config_home = sys.argv[1]
     harness = sys.argv[2]