## Summary
This PR narrows original image detail handling to a single opt-in
feature:
- `image_detail_original` lets the model request `detail: "original"` on
supported models
- Omitting `detail` preserves the default resized behavior
The model only sees `detail: "original"` guidance when the active model
supports it:
- JS REPL instructions include the guidance and examples only on
supported models
- `view_image` only exposes a `detail` parameter when the feature and
model can use it
The image detail API is intentionally narrow and consistent across both
paths:
- `view_image.detail` supports only `"original"`; otherwise omit the
field
- `codex.emitImage(..., detail)` supports only `"original"`; otherwise
omit the field
- Unsupported explicit values fail clearly at the API boundary instead
of being silently reinterpreted
- Unsupported explicit `detail: "original"` requests fall back to normal
behavior when the feature is disabled or the model does not support
original detail
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