feat(media): add Atlas image generation skill

[binyangzhu000-sudo]

Summary

• add an optional atlas-image-generation media skill with live model discovery and schema validation
• add a stdlib-only helper that submits a generation POST once, bounds result GET retries, and verifies the downloaded image
• add focused tests for catalog filtering, schema validation, reserved fields, retry boundaries, and endpoint discovery
• update the existing media inventory and skill count

Why

The repository already provides stock-image sourcing and external media API workflows. This adds an original-image route for users who explicitly choose Atlas Cloud without changing any existing default or stock-image workflow.

Validation

• python3 -m unittest discover -s agent-skills/media/atlas-image-generation/tests -p 'test_*.py' -v (7 passed)\n- python3 -m py_compile ...\n- CLI --help, live --list-models, and schema-backed --dry-run\n- one real generation POST after fetching the live catalog and model schema; bounded polling completed and produced a verified non-empty 1024x1024 PNG\n- git diff --check plus staged secret and absolute-path scans\n\n## Safety\n\n- API keys remain environment-only and are never logged\n- POST requests are never automatically retried\n- only transient GET requests use bounded backoff\n- model IDs and request fields are resolved from the live catalog and schema\n\nAI-assisted: Codex prepared the implementation and tests; the final diff and live validation results were reviewed before submission.