feat: use Codex Images endpoints

[ningzimu]

Summary

• Route Codex OAuth image generation and edits through Codex Images endpoints.
• Align gpt-image-2 defaults, validation, and docs with the supported parameter surface.
• Consolidate image API references and update README warnings.

Verification

• python3 -m py_compile skills/codex-gpt-image/scripts/codex_gpt_image.py
• python3 /Users/ningzimu/.codex/skills/.system/skill-creator/scripts/quick_validate.py skills/codex-gpt-image
• git diff --check
• Manual smoke tests for generate/edit PNG, JPEG, WebP, mask, and explicit sizes.

Notes

• This uses the Codex Images backend, not the recommended official API-key integration path.