[INSTRUMENTOR] Add support for Cohere

[Sigmabrogz]

Description

Add auto-instrumentation support for Cohere LLM API.

Why

Cohere is a popular LLM provider, especially for enterprise use cases. Adding support would help more developers debug their agents.

Implementation Guide

1. Create src/agentdbg/instrumentors/cohere_instrumentor.py
2. Wrap cohere.Client.chat() and cohere.Client.generate() methods
3. Extract token counts from response for cost tracking
4. Add to auto_instrument() in __init__.py
5. Add tests in tests/unit/test_instrumentors.py
6. Add example in examples/cohere_agent.py

Resources

• Cohere Python SDK
• Look at openai_instrumentor.py for reference

Good First Issue

This is a great first contribution! The pattern is well-established in existing instrumentors.