Replaces the stateless ask-once backend rule with a 4-step resolution
(current-request override > saved preference > auto-select > ask) and
adds a single `preferred_image_backend` field (auto | ask | <backend-id>)
to every image-consuming skill's EXTEND.md schema. Runtime-native tools
(Codex `imagegen`, Hermes `image_generate`, ...) win by default; absent
field equals `auto` so existing user EXTEND.md files stay valid with no
schema version bump.
Each image-consuming skill also gains a top-level `## Changing
Preferences` section as a first-class surface for pinning a backend and
editing common one-line preferences.
Applies across:
- baoyu-infographic (reverts in-progress two-field image_backend_mode design)
- baoyu-comic
- baoyu-cover-image
- baoyu-image-cards
- baoyu-article-illustrator
- baoyu-slide-deck
- baoyu-xhs-images
- docs/image-generation-tools.md (canonical author-side doc)
Jim Liu 宝玉