The codex image provider's fail-fast gate required an `imagegenext` row in
`codex features list`. Codex CLI 0.145 renamed that flag to `image_generation`
and dropped the old row, so the gate rejected every up-to-date CLI:
media-use: codex image upsell unavailable: codex imagegenext unavailable
(upgrade Codex CLI)
`resolve --type image --provider codex` returned that without ever attempting
a render, on hosts where image generation works fine. Codex itself now warns
`[features].imagegenext is deprecated. Use [features].image_generation`.
Resolve the flag name from the feature list instead of hardcoding it, and pass
whichever name the installed CLI exposes to `--enable`. The old name is
preferred when present, since CLIs that still list it reject the new one.
`codexUnavailableReason()` now returns `{ flag }` / `{ reason }` so the
resolved name reaches the exec.
Verified against codex-cli 0.145.0: `resolve --type image --provider codex`
generates and freezes an image. New unit tests pin both flag layouts so a
future rename fails loudly instead of silently disabling the provider.
