feat(setup): add Atlas Cloud provider preset

[binyangzhu000-sudo]

Summary

• add Atlas Cloud to the interactive provider catalog through its OpenAI-compatible endpoint
• prefill qwen/qwen3.8-max while retaining live model discovery and verification
• document the provider option in both English and Chinese READMEs

Why

Echo Agent already supports OpenAI-compatible providers, but Atlas Cloud users must currently choose the custom endpoint path and enter every value manually. This preset makes the existing setup workflow discoverable and validates the selected model before saving the configuration.

Changes

• register atlascloud with https://api.atlascloud.ai/v1, ATLASCLOUD_API_KEY, and the live /models catalog
• add setup and catalog regression coverage
• keep runtime routing on the existing OpenAI-compatible provider implementation

Test Plan

• pytest tests/cli/setup/test_providers.py tests/cli/setup/test_model_section.py tests/cli/setup/test_model_verify.py -q (30 passed)
• ruff check .
• Dashboard TypeScript/Vite build and Python wheel build
• Live catalog discovery (129 models, default model present) and live qwen/qwen3.8-max verification through Echo Agent
• Full suite exercised: 5961 passed, 51 skipped; 12 unrelated failures were reproduced unchanged on master (browser URL guard/timeout tests and install-script classifier tests in this local environment)

Risk and rollback

The change only extends the setup catalog and documentation; runtime requests continue through the existing OpenAI-compatible provider. Reverting this commit removes the preset without affecting saved custom-provider configurations.

Checklist

• No changes to protected areas (src/agent/, src/session/, src/providers/) without prior discussion
• No hardcoded API keys, file paths, or credentials
• Code follows CONTRIBUTING.md
• Documentation updated