Feature: add subtraction command

[nholthaus]

Problem

The built-in math commands currently cover addition and multiplication, but not subtraction. This leaves a basic arithmetic gap and forces scripts to work around simple numeric operations.

Specification

Add a subtraction command to the built-in command set.

Acceptance criteria:

• Scripts can subtract one integer from another and store the result in an output variable.
• The command follows the existing argument style used by the other math commands.
• Variable operands are supported through the current dereferencing behavior.
• Invalid argument counts or non-integer inputs fail with a clear error message.

Motivation

This would round out the basic arithmetic surface and provide a small, self-contained feature for contributors.