feat: add Atlas Cloud provider preset

[binyangzhu000-sudo]

Summary

• add atlascloud as a built-in OpenAI-compatible provider preset
• default to https://api.atlascloud.ai/v1 and deepseek-ai/deepseek-v4-pro
• support the shorter atlas alias while keeping explicit base URL, model, and API key overrides unchanged
• expose the preset through the existing CLI/GUI provider catalog

Validation

• cargo test -p deeptide-host provider
• cargo check -p deeptide-cli
• cargo fmt --all -- --check
• git diff --check
• Atlas Cloud catalog probe: default model present in the public model catalog
• Atlas Cloud live probe: POST /v1/chat/completions returned HTTP 200 with response content

Scope

No README, documentation, logo, sponsor, credits, or partner-promotion changes. No credentials are included.