fix: throw error on division by zero in calculator

[quantcode-agent]

Summary

• Fixes the division-by-zero bug in src/calculator.ts by adding a guard clause that throws an Error("Division by zero") when the divisor is 0
• Adds test coverage for both normal division and the division-by-zero error case

Changes

• src/calculator.ts: Added if (b === 0) throw new Error("Division by zero") at the top of the divide function
• test/calculator.test.ts: Added test cases verifying divide(10, 2) === 5 and divide(10, 0) throws

Assumptions

• Throwing an Error is the appropriate behavior (vs returning null/NaN/Infinity)
• The error message "Division by zero" is sufficient for consumers of this function

All 5 tests pass.