feat(llm): add Atlas Cloud provider configuration

Summary

• add ATLASCLOUD_* and ATLAS_CLOUD_* environment aliases with the default OpenAI-compatible base https://api.atlascloud.ai/v1
• route Atlas Cloud credentials through the existing LiteLLM generate, stream, and summarize paths while preserving generic OPENAI_* precedence
• pass both alias forms through Docker Compose and add an optional .env.example configuration using openai/deepseek-ai/deepseek-v4-pro
• cover environment precedence, key masking, endpoint routing, and non-OpenAI provider isolation

Validation

• pytest -q tests/unit/config tests/unit/llm (135 passed)
• Black check on changed Python files
• Ruff check on changed Python files
• python -m compileall -q pentestagent/config pentestagent/llm
• docker compose config -q
• git diff --check and git diff --cached --check
• Atlas Cloud live catalog: authenticated /v1/models returned HTTP 200 with 121 models; confirmed deepseek-ai/deepseek-v4-pro, deepseek-ai/deepseek-v4-flash, and qwen/qwen3.5-flash

Documentation scope

No README changes. .env.example and Docker Compose only add optional technical configuration; no sponsor, logo, credits, or partner promotion.