Add a calculator.py script with basic math operations

[dzianisv]

Create a Python script called calculator.py at the repository root that implements basic math operations (add, subtract, multiply, divide). It should work from the command line: python calculator.py add 2 3 should output 5.

[opencode-agent]

Created PR #686

opencode session  |  github run

[opencode-agent]

Created PR #687

opencode session  |  github run