fix: handle division by zero in calculator

[quantcode-agent]

Summary

• Added a guard clause in src/calculator.ts divide() that throws an error when the divisor is zero, preventing JavaScript's default Infinity result
• Added tests in test/calculator.test.ts for normal division and division-by-zero error handling

Assumptions

• The appropriate behavior for division by zero is to throw an Error with message "Division by zero is not allowed"
• All existing tests continue to pass (5/5 passing)