Generate bitmap assets in the coding workflow using Codex's imagegen Skill

# Introduction to OpenAI Codex System-level Skill `imagegen` This skill enables generation and editing of bitmap assets including photos, illustrations, mockups and sprites within coding conversations via either the built-in `image_gen` command or an explicit CLI. Details including default auto-installation location, two operating modes, prompt templates, save path conventions, applicable boundaries and official CLI examples are provided to facilitate directly integrating visual assets into project repositories.