fix: make -m/--mode a required argument

When no -m flag is provided, args.mode is None (since action='append'
defaults to None). This causes a TypeError in print_summary() which
iterates over args.mode.

Mark the argument as required=True so argparse emits a clear usage
error instead of crashing with an unhelpful traceback. Also remove
"[default]" from the help text for mode 0 since there is no longer
a default.

Fixes #32

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: themavik

src/flexidot/utils/args.py

@@ -82,7 +82,8 @@ def parse_args():
         '--mode',
         action='append',
         choices=['0', '1', '2'],
-        help='Mode of FlexiDot dotplotting. 0 = self [default], 1 = paired, 2 = poly (matrix with all-against-all dotplots). Call -m multiple times to run multiple modes.',
+        required=True,
+        help='Mode of FlexiDot dotplotting. 0 = self, 1 = paired, 2 = poly (matrix with all-against-all dotplots). Call -m multiple times to run multiple modes.',
     )
     parser.add_argument(
         '-t',