feat(providers): add optional Atlas Cloud provider

[binyangzhu000-sudo]

Summary

Add Atlas Cloud as an optional provider in the existing provider registry and interactive setup flow. This follows the repository's adding-provider skill and leaves existing provider behavior and defaults unchanged.

• Reuse the OpenAI-compatible request/response implementation with separate ATLASCLOUD_API_KEY credentials.
• Preserve full catalog model IDs such as openai/gpt-4.1-mini and support ATLASCLOUD_BASE_URL without mutating shared configuration.
• Wire Atlas Cloud in the setup menu to the atlascloud registry key and document configuration in the README.
• Add standard provider error tests, request/model/usage assertions, credential isolation, custom-endpoint isolation, and setup persistence coverage.

Validation

• uv run --locked --extra dev pytest -q: 3096 passed, 47 integration tests deselected.
• Ruff check and format: passed across source and tests.
• uv run --locked mypy src/gac: passed, 107 source files.
• README Prettier and markdownlint checks: passed.
• uv build: wheel and source distribution built successfully.
• One live smoke through gac -sy --signoff --one-liner --model atlascloud:openai/gpt-4.1-mini generated this signed commit, using an isolated temporary HOME and GAC_RETRIES=1. HTTP 200, 5133 prompt tokens and 21 completion tokens. No generation request was retried.

Full make lint encounters pre-existing Prettier failures in docs/en/PROVIDERS.md; the full markdownlint run reports 13 existing issues in that same file. It is byte-for-byte identical to the base revision and is not modified here. Other providers' paid integration tests were not run.

This change was prepared with AI assistance; the diff, test results, and live request were checked before submission.

[thomwebb]

hi @binyangzhu000-sudo, thank you for the pr! can you get model conflicts resolved and ci passing?