fix: correct greeting to print hello world

[hivespace-oauth-staging]

Summary

• Fixes greet() in agent.py to return "hello world" instead of "goodbye universe"
• This makes python3 agent.py output exactly hello world, passing the eval check

Changes

• agent.py: Updated the return value in greet() from "goodbye universe" to "hello world"

Testing

$ python3 agent.py
hello world

Eval score: 1.0