fix: handle division by zero in calculator

[quantcode-agent]

Summary

• Added a guard clause in src/calculator.ts divide function that throws an Error("Division by zero is not allowed") when the divisor is zero
• Added test cases in test/calculator.test.ts for both normal division and division-by-zero error handling

Assumptions

• Throwing an error is the appropriate behavior for division by zero (rather than returning Infinity, NaN, or a custom result type)
• The error message "Division by zero is not allowed" is descriptive enough for consumers

Testing

All 5 tests pass (including the 2 new division-related tests).