[codex] Fix Windows image_gen fallback for sprites

[luyh7]

Summary

• Adds a save_imagegen_result.py helper that can persist Codex image_generation_call.result base64 output as a local image file.
• Updates the $generate2dsprite raw-image handoff contract to avoid stale generated_images files and guessed /mnt/data/... paths.
• Documents that this fallback is for the Windows/Codex Desktop-only failure mode where image_gen renders in chat but does not write a current PNG under the expected generated-images directory.

Root Cause

This fixes #4. The bug only shows up on Windows/Codex Desktop sessions: the generated image is visible in the conversation, but no matching current file appears under $CODEX_HOME/generated_images or ~/.codex/generated_images. The sprite processor then receives no valid --input PNG and stops before producing sheet-transparent.png, frame PNGs, or GIF output.

The new contract falls back to the current Codex session JSONL / image_generation_call.result base64 payload, decodes it into the run-local generated_images/ folder, and then feeds that stable path into the existing processor.

Validation

• python3 -m py_compile skills/generate2dsprite/scripts/save_imagegen_result.py
• Fixture session JSONL containing an unrelated data:image/... tool output plus an image_generation_call.result; helper decoded the latest image call into a PNG.
• python3 skills/generate2dsprite/scripts/generate2dsprite.py process --input <decoded_png> --target creature --mode idle ... produced sheet-transparent.png, four frame PNGs, animation.gif, and pipeline-meta.json.