feat: add Atlas Cloud provider

[binyangzhu000-sudo]

Summary

• add a first-class Atlas Cloud provider for the OpenAI-compatible Chat Completions API
• resolve ATLASCLOUD_API_KEY and default to https://api.atlascloud.ai/v1
• add provider loading/configuration tests, an example agent, and provider documentation

Validation

• bundle exec ruby bin/test test/providers/atlas_cloud/provider_loading_test.rb test/features/provider_test.rb (39 runs, 58 assertions)
• bundle exec rubocop --config .rubocop.yml (375 files, no offenses)
• npm run docs:build
• gem build activeagent.gemspec
• real Atlas Cloud request through generate_with :atlas_cloud using qwen/qwen3.8-max

The Rails 8 full suite completed with 1,295 runs, 0 failures, and 30 errors. The same 30 RubyLLM live-key/API compatibility errors reproduce on the unmodified base commit (1,291 runs, 0 failures, 30 errors).