feat(paths): remove SLOP_CONFIG env-var fallback; add --config CLI flag (FR2)

src/models.py

@@ -191,8 +191,6 @@ def _save_config_to_disk(config: dict[str, Any]) -> None:
  # only when the user actually saves config.
  import tomli_w
  config = _clean_nones(config)
- sys.stderr.write(f"[DEBUG] Saving config. Theme: {config.get('theme')}\n")
- sys.stderr.flush()
  with open(get_config_path(), "wb") as f:
   tomli_w.dump(config, f)