feat(design): add optional Atlas Cloud logo provider

[binyangzhu000-sudo]

What does this PR change?

Adds Atlas Cloud as an explicit opt-in provider for the logo generator while preserving Gemini as the default. The integration uses the asynchronous image API with a single generation POST, bounded result polling, HTTPS media validation, credential-free downloads, CLI flags, documentation, and offline contract tests.

Why?

Users with an Atlas Cloud key can generate logo variants without installing the Gemini SDK, while existing Gemini workflows and defaults remain unchanged.

Validation

• python3 -m pytest .claude/skills -q (219 passed)
• npm run verify:data
• npm run typecheck
• npm run build
• Ruff checks for the changed Python files
• Live model catalog and schema verification
• Live Atlas generation reached completed; the no-credential media downloader was separately verified with a live Atlas-hosted PNG

Checklist

• Source changes are in .claude/skills/design/, the source of truth for bundled design sub-skills
• Ran npm run sync:assets && npm run check:assets in cli/
• Added offline tests under .claude/skills/design/scripts/logo/tests/
• Commit message follows Conventional Commits
• This PR targets a feature branch, not main

[clark-cant]

LGTM — solid contribution. Atlas Cloud integration is well-structured with proper security measures (SSRF-safe redirects, public URL validation, credential-free media downloads), comprehensive offline contract tests, and clean backward compatibility (Gemini remains default).

Approved findings:

• Security: SSRF protection on redirects and media download is correctly implemented
• Tests: Contract tests cover polling, timeout, retry, and API key isolation
• Docs: SKILL.md, references, and CLI help all updated consistently
• Code quality: Proper error handling, type hints, clean provider abstraction

Note for maintainers: No CI checks are configured on this repository — the validation evidence in the PR body (pytest 219 passed, npm verify/typecheck/build) was self-reported. Consider adding CI to verify future PRs automatically.

Reviewed by github-maintain cron