feat: Add Unix pipe support for draw command

[thehale]

This change allows the tnoodle draw command to read a scramble string from standard input if the -s or --scramble parameter is not explicitly provided. This enables piping the output of tnoodle scramble directly to tnoodle draw.

Key changes:

• Modified DrawCommand.java to make the scramble parameter optional and to read from System.in if the scramble is not given as an argument.
• Added robust error handling for cases such as empty stdin or invalid scramble input.
• Introduced DrawCommandTest.java with unit tests:
  • testDrawCommand_readsFromStdin: Verifies reading a scramble directly from stdin.
  • testFullPipeWorkflow_scrambleToDraw: Verifies the complete workflow of piping tnoodle scramble output to tnoodle draw.
• Updated README.md to document the new piping feature with an example: tnoodle scramble -p three | tnoodle draw -p three > output.svg.

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