Add Atlas Cloud provider support

[binyangzhu000-sudo]

Summary

• add Atlas Cloud as a built-in OpenAI-compatible provider with ATLASCLOUD_API_KEY, https://api.atlascloud.ai/v1, and live-verified chat model defaults
• wire the provider into setup, catalog status, routing labels, conservative capabilities, and OpenAI-compatible summary/structured chat paths
• add provider docs and focused coverage for normalization, setup status, and chat-completions routing

Verification

• bun install --frozen-lockfile
• bunx tsc --noEmit
• bun test packages/runtime/src/provider.integration.test.ts packages/runtime/src/provider.test.ts packages/runtime/src/runtime/setup-api.test.ts --test-name-pattern 'atlascloud|Atlas Cloud|providerConfigured reflects|non-azure providers|providerDisplayLabel|providerReauth|extraBody|generateTaskSummary'
• git diff --check
• Atlas live /v1/models returned 118 models and included qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro

Notes

• README changes are limited to the existing provider list/setup examples.
• No sponsor, logo, credits, partner, or promotional sections were added.