Add Atlas Cloud LLM provider

[binyangzhu000-sudo]

Summary

• add an Atlas Cloud OpenAI-compatible LLM provider that reads ATLASCLOUD_API_KEY / ATLAS_CLOUD_API_KEY
• allow evolver_model strings such as atlascloud:qwen/qwen3.5-flash across the default LLM factories and unified evolver route
• extend OpenAIProvider with optional base_url support and add focused provider resolution tests

Validation

• python3 -m pytest tests/test_atlascloud_provider.py -q
• python3 -m compileall -q agent_evolve tests/test_atlascloud_provider.py
• git diff --check
• Atlas live /v1/models check confirmed qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro

README

• Updated the existing LLM Provider examples row to include Atlas Cloud. No sponsor/logo/credits/partner block added.