Add Atlas Cloud LLM provider preset

[binyangzhu000-sudo]

Summary

• add an atlascloud LLM provider preset for EverOS OpenAI-compatible chat completions
• default Atlas Cloud to https://api.atlascloud.ai/v1 and qwen/qwen3.5-flash while preserving explicit model/base URL overrides
• allow ATLASCLOUD_API_KEY / ATLAS_CLOUD_API_KEY as convenience env fallbacks
• update config templates/docs and focused settings tests

Validation

• uv run --python 3.12 pytest tests/unit/test_config/test_settings.py tests/unit/test_component/test_llm/test_factory.py -q
• uv run --python 3.12 ruff check src/everos/config/settings.py tests/unit/test_config/test_settings.py
• uv run --python 3.12 python -m compileall -q src/everos/config/settings.py tests/unit/test_config/test_settings.py tests/unit/test_component/test_llm/test_factory.py
• git diff --check
• confirmed qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro in the live Atlas Cloud model catalog