Add Atlas Cloud model shortcut

[binyangzhu000-sudo]

Summary

• Add atlascloud:, atlas-cloud:, and atlas: model shortcuts that route through the existing OpenAI-compatible custom model path.
• Default Atlas Cloud shortcuts to https://api.atlascloud.ai/v1, with ATLASCLOUD_API_BASE / ATLASCLOUD_BASE_URL / ATLAS_CLOUD_API_BASE / ATLAS_CLOUD_BASE_URL overrides.
• Map ATLASCLOUD_API_KEY and ATLAS_CLOUD_API_KEY into the existing OpenAI-compatible API key flow, and cover the shortcut behavior with focused loader tests.

No README changes.

Validation

• npm ci --ignore-scripts
• npm run build
• node --test test/loader-atlascloud.test.ts --experimental-strip-types
• npm test
• git diff --check
• Atlas Cloud live model catalog returned 374 visible models and confirmed qwen/qwen3.5-flash plus deepseek-ai/deepseek-v4-pro

[Nivesh353]

@binyangzhu000-sudo We already support openrouter: as a native provider — 272 models currently registered with real pricing built in (no custom-model workaround needed, unlike this shortcut). That already covers most of what's being added here. Closing as duplicate — let me know if there's a specific model or use case OpenRouter doesn't cover and we can revisit.

[shreyas-lyzr]

Solid addition. The implementation is clean, well-contained, and the test coverage is good. Two minor things worth addressing in a follow-up, though neither blocks merge.

Atlas Cloud's LLM models are fully reachable via openrouter: today with no workaround. We'd rather invest in a generic named-endpoint mechanism than accumulate per-vendor shortcuts