fix: handle division by zero in calculator

[quantcode-agent]

Summary

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

Assumptions

• Throwing an error is the appropriate behavior for division by zero (rather than returning null/NaN/Infinity)
• The error message "Division by zero" is sufficient for consumers to identify the issue

All 5 tests pass.