fix: handle division by zero in calculator

[quantcode-agent]

Summary

• Fixes division-by-zero bug in src/calculator.ts where divide(x, 0) would return Infinity instead of throwing an error
• The divide() function now throws Error("Cannot divide by zero") when the divisor is zero
• Added tests for normal division and the divide-by-zero error case

Changes

• src/calculator.ts: Added guard clause at the top of divide() to check for zero divisor
• test/calculator.test.ts: Added test for successful division and test verifying the error is thrown on divide-by-zero

Testing

All 5 tests pass (bun test).