feat: add greet CLI with tests

tests/test_cli.py

@@ -0,0 +1,10 @@
+from click.testing import CliRunner
+
+from py_demo.cli import main
+
+
+def test_greet_outputs_hello():
+    runner = CliRunner()
+    result = runner.invoke(main, ["greet", "World"])
+    assert result.exit_code == 0
+    assert "Hello, World!" in result.output