Add Atlas Cloud provider defaults

[binyangzhu000-sudo]

Summary

• add Atlas Cloud as a built-in OpenAI-compatible provider type with defaults for https://api.atlascloud.ai/v1 and qwen/qwen3.5-flash
• wire Atlas Cloud through config/env loading, secret masking, startup and DB provider registration, CLI/setup provider selection, and model listing fallback
• document the provider in the existing provider table without changing the root README or adding sponsor/logo/partner content

Validation

• git diff --check
• static self-review of all Atlas Cloud registration paths
• live Atlas Cloud /v1/models check returned 118 models and confirmed qwen/qwen3.5-flash plus deepseek-ai/deepseek-v4-pro

Note: Go tooling is not available in this local environment (go/gofmt are not on PATH), so I could not run Go tests locally.