[codex] Update imagegen system skill

[vb-openai]

Summary

Updates the embedded imagegen system skill with the latest gpt-image-2 routing, CLI, and documentation guidance.

The skill now keeps normal image generation/editing on the built-in image_gen path, makes gpt-image-2 the default CLI fallback model, and treats simple transparent-image requests as built-in-first: generate on a flat chroma-key background, then remove the key locally with scripts/remove_chroma_key.py to produce an alpha PNG/WebP.

True/native transparency is now a separate confirmed fallback path. CLI gpt-image-1.5 --background transparent --output-format png is reserved for explicit native-transparency asks, subjects too complex for chroma-key removal, or failed local alpha validation, because gpt-image-2 does not support background=transparent.

Details

• Update SKILL.md routing guidance for built-in vs CLI fallback behavior.
• Keep “batch” requests on the built-in path unless the user explicitly asks for CLI/API/model controls.
• Add the built-in-first transparent workflow:
  • choose a subject-safe chroma-key color
  • prompt for one perfectly flat solid background
  • prohibit that key color inside the subject
  • run scripts/remove_chroma_key.py --auto-key border
  • validate alpha output before using it
• Add scripts/remove_chroma_key.py for chroma-key alpha extraction, soft matte, despill, edge contraction, and PNG/WebP output.
• Clarify that CLI gpt-image-1.5 native transparency requires user confirmation unless explicitly requested.
• Update CLI/API docs for gpt-image-2:
  • default CLI fallback model
  • flexible size validation
  • 3840x2160 / 2160x3840 4K guidance
  • quality values including auto
  • unsupported background=transparent
  • unsupported input_fidelity
• Update prompt/reference docs and sample prompts with the new transparent-image workflow.

Validation

• cargo test -p codex-skills
• git diff --check
• Manual CLI dry-runs for:
  • default gpt-image-2 payload
  • 4K size acceptance
  • invalid size rejection
  • transparent background rejection on gpt-image-2
  • transparent background acceptance on gpt-image-1.5
  • input_fidelity rejection on gpt-image-2
  • chroma-key removal workflow

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[gabrielchua-oai]

I have read the CLA Document and I hereby sign the CLA

[gabrielchua-oai]

recheck

[gabrielchua-oai]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 13dba916a1

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".