Remove invalid required=True from click.argument with nargs=-1 (#17)

* Initial plan

* Fix click.argument usage by removing required=True and adding validation

Co-authored-by: ioggstream <1140844+ioggstream@users.noreply.github.com>

* Add required=True to --output option to prevent None value

Co-authored-by: ioggstream <1140844+ioggstream@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ioggstream <1140844+ioggstream@users.noreply.github.com>

Author: Copilot

d3fendtools/as_mermaid/__main__.py

@@ -12,9 +12,11 @@
 
 
 @click.command()
-@click.argument("files", nargs=-1, type=click.Path(exists=True), required=True)
-@click.option("--output", "-o", help="Output markdown file")
+@click.argument("files", nargs=-1, type=click.Path(exists=True))
+@click.option("--output", "-o", required=True, help="Output markdown file")
 def main(files, output):
+    if not files:
+        raise click.UsageError("At least one file is required")
     output_md = Path(output)
     resource_paths = []
     for file in files: