interface-design 2026.6.12

What changed

• Added Codex-compatible skill metadata at .claude/skills/interface-design/agents/openai.yaml.
• Added a Codex image generation workflow reference for direction boards, UI reference mockups, screenshot paintovers, and project-bound raster assets.
• Updated SKILL.md to use the imagegen workflow when useful for visual direction and post-build critique, while keeping Claude Code compatibility.
• Reworked README installation guidance around the skills.sh installer, including Claude Code and Codex-specific commands.
• Bumped Claude plugin metadata to 2026.6.12.1241.

Validation

• quick_validate.py passes for the source skill.
• Plugin JSON files parse successfully.
• git diff --check HEAD passes.