Clarify js_repl image emission and encoding guidance

[fjord-oai]

Summary

This updates the js_repl prompt and docs to make the image guidance less confusing.

What changed

• Clarified that codex.emitImage(...) adds one image per call and can be called multiple times to emit multiple images.
• Reworded the image-encoding guidance to be general js_repl advice instead of ImageDetailOriginal-specific behavior.
• Updated the guidance to recommend JPEG at about quality 85 when lossy compression is acceptable, and PNG when transparency or lossless detail matters.
• Mirrored the same wording in the public js_repl docs.