fix: handle division-by-zero in calculator

[quantcode-agent]

Summary

• Added a guard in src/calculator.ts divide() that throws an Error("Division by zero is not allowed") when the divisor is zero, preventing Infinity/NaN results.
• 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 a sentinel value).
• All existing tests continue to pass (verified: 5/5 pass).