Add a minimal 'hello world' agent example

[rajkumar42]

Summary

Add the simplest possible agent example — a "hello world" that demonstrates the absolute minimum code needed to create and run an OpenSymbolicAI agent. This is the first thing new users should see.

What needs to happen

• Create hello_world/ directory with a single-file agent
• The agent should do something simple (e.g., answer a question, greet the user)
• Keep it under 20 lines of code
• Add a README.md explaining each line
• Ensure it runs with python hello_world/main.py

Example

from opensymbolicai import Agent

agent = Agent(goal="Greet the user and tell them a fun fact")
result = agent.run()
print(result)

Acceptance criteria

• Example runs in under 10 seconds
• Code is under 20 lines
• README explains what each part does
• A complete beginner can follow it