Always enable original image detail on supported models

[fjord-oai]

Summary

This PR removes image_detail_original as a runtime experiment and makes original image detail available whenever the selected model supports it.

Concretely, this change:

• drops the image_detail_original feature flag from the feature registry and generated config schema
• makes tool-emitted image detail depend only on ModelInfo.supports_image_detail_original
• updates view_image and code_mode/js_repl image emission to use that capability check directly
• removes now-redundant experiment-specific tests and instruction coverage
• keeps backward compatibility for existing configs by silently ignoring a stale features.image_detail_original entry

The net effect is that detail: "original" is always available on supported models, without requiring an experiment toggle.

[fjord-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Nice work!

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