Reject directories as edit targets

src/mirro/main.py

@@ -491,6 +491,10 @@ def main():
     target = Path(file_arg).expanduser().resolve()
     backup_dir = Path(args.backup_dir).expanduser().resolve()
 
+    if target.is_dir():
+        print(f"'{target}' is a directory!")
+        return 1
+
     # Permission checks
     parent = target.parent
     if target.exists() and not os.access(target, os.W_OK):

tests/test_mirro.py

@@ -174,6 +174,17 @@ def test_main_missing_argument(capsys):
     )
 
 
+def test_main_rejects_directory(tmp_path, capsys):
+    target = tmp_path / "dir"
+    target.mkdir()
+
+    with patch("sys.argv", ["mirro", str(target)]):
+        result = mirro.main()
+
+    assert result == 1
+    assert f"'{target}' is a directory!" in capsys.readouterr().out
+
+
 # ============================================================
 # main: unchanged file
 # ============================================================