fix: handle division-by-zero in calculator

[quantcode-agent]

Summary

• Adds a guard in src/calculator.ts divide() that throws an Error("Division by zero is not allowed") when the divisor is 0
• Adds tests in test/calculator.test.ts for 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 Infinity, NaN, or a result type)
• The existing test file structure and test runner (bun test) are the correct conventions for this repo

All 5 tests pass.