fix: handle division by zero in divide()

[quantcode-agent]

Summary

• Fixed the division-by-zero bug in src/calculator.ts by throwing Error("Division by zero") when the divisor is 0
• Added tests for divide() in test/calculator.test.ts covering normal division and the zero-divisor error case

Changes

• src/calculator.ts: Added if (b === 0) throw new Error("Division by zero") guard in divide()
• test/calculator.test.ts: Added divide to imports and a describe("divide") block with 2 tests

Test results

All 5 tests pass (bun test).