feat: support Atlas Cloud environment configuration

[binyangzhu000-sudo]

Summary

• use ATLASCLOUD_API_KEY as an opt-in credential when no explicit API_KEY is configured
• select the Atlas Cloud OpenAI-compatible base URL and namespaced DeepSeek V4 model by default
• preserve explicit user, project, and DEEPCODE_* settings precedence
• recognize the namespaced model for thinking-mode and multimodal capability defaults

Validation

• npm run generate && npm run check
• npm test (596 passed, 1 skipped)
• Atlas Cloud model discovery through createOpenAIClient() (123 models; default model present)
• Atlas Cloud chat completion through createOpenAIClient() (non-empty response)
• git diff --check

No README, public documentation, logo, dependency, or lockfile changes.

[binyangzhu000-sudo]

Resolved the merge conflict with current main in follow-up commit 7084a7b9.

The resolution preserves both the Atlas Cloud settings tests and the new upstream context-window tests. The online PR diff remains limited to the original four core files.

Validation:

• Focused settings tests: 35 passed, 0 failed
• Core test suite: 281 passed, 1 skipped, 0 failed
• All workspace typechecks passed
• ESLint, Prettier, conflict-marker, and diff-scope checks passed

The exact-head CI workflow currently shows action_required, so it needs maintainer approval to execute for this fork PR.