fix: handle division by zero in calculator

[quantcode-agent]

Summary

• Added a guard clause in src/calculator.ts divide() function to throw an Error("Division by zero") when the divisor is zero, instead of returning Infinity
• Added tests in test/calculator.test.ts verifying both normal division and the division-by-zero error case

Assumptions

• Throwing an Error is the appropriate behavior for division by zero (rather than returning null, NaN, or a result object)
• The error message "Division by zero" is sufficient for consumers to identify the issue

All 5 tests pass.