Add Cohere adapter

[hidai25]

Summary

Add a CohereAdapter so EvalView can test agents built on the Cohere API.

Models to support

• command-r-plus
• command-r
• command

How to implement

• Create evalview/adapters/cohere_adapter.py
• Follow the pattern of existing adapters (e.g. evalview/adapters/http_adapter.py)
• Auth via COHERE_API_KEY environment variable
• Register the adapter in _create_adapter() in evalview/cli.py

Config example (what users will write)

adapter: cohere
model: command-r-plus

Acceptance criteria

• adapter: cohere in .evalview/config.yaml works end-to-end
• Auth error gives a clear message when COHERE_API_KEY is missing
• Follows the AgentAdapter base class interface

Good first issue notes

One file to create, one line to add in cli.py. Look at any existing adapter for the full pattern. The Cohere Python SDK (cohere) is well-documented.