chore: remove gng, move demo out of lib init

tests/test_no_gng.py

@@ -0,0 +1,24 @@
+import subprocess
+import sys
+
+
+def test_gng_removed():
+    try:
+        import agentmsgs.utils  # noqa: F401
+
+        assert False, "utils should be deleted"
+    except ModuleNotFoundError:
+        pass
+    import agentmsgs
+
+    assert hasattr(agentmsgs, "main") or True
+
+
+def test_import_does_not_print():
+    result = subprocess.run(
+        [sys.executable, "-c", "import agentmsgs"],
+        capture_output=True,
+        text=True,
+    )
+    assert result.stdout == "", f"import printed: {result.stdout!r}"
+    assert result.returncode == 0