Add CrewAI instrumentation support

[Cubiczan]

AgentForge currently supports OpenAI and LangChain tracing. Add CrewAI support via a callback handler similar to the existing AgentForgeCallbackHandler.

What to do:

• Study how CrewAI handles agent execution steps
• Create agentforge/crewai.py with a callback/instrumentation class
• Capture: task execution, tool calls, LLM calls, agent decisions
• Add example in examples/crewai_agent.py

Getting started:

1. Clone the repo: git clone https://github.com/Cubiczan/agentforge.git
2. Install: pip install -e .
3. Check agentforge/langchain.py for reference implementation

Tags: good first issue