feat: add optional Atlas asset generation skill

[binyangzhu000-sudo]

Summary

Adds /asset-generate, a focused bridge between approved /asset-spec prompts and a single reviewable game-art preview generated with Atlas Cloud as an optional provider.

Type of Change

• New agent
• New skill
• New hook or rule
• Bug fix
• Documentation improvement
• Other:

Changes

• add a collaborative /asset-generate workflow that asks before every billable request and leaves production asset decisions to the user
• add a dependency-free Python helper that validates the live model catalog/schema, never retries generation POSTs, bounds prediction polling, and keeps outputs inside the project
• add deterministic unit coverage, a behavioral test spec, catalog registration, and command-reference updates

Testing

• python3 -m unittest discover -s .claude/skills/asset-generate/tests -v (10 tests passed)
• git diff --check
• live model catalog and schema validation for google/nano-banana-pro/text-to-image-developer
• one real 1k generation request reached a completed JPEG output; the smoke exposed a requested-extension mismatch, which was fixed with format-signature detection and a focused regression test. I did not submit a second billable request.

Checklist

• I've tested this in a Claude Code session
• New agents include the Collaboration Protocol section
• New skills use the subdirectory format (.claude/skills/<name>/SKILL.md)
• Reference docs are updated (agent-roster, skills-reference, hooks-reference, rules-reference)
• Hooks use grep -E (POSIX) and fail gracefully without jq/python
• No hardcoded paths or platform-specific assumptions