refactor: unify image-backend resolution with preferred_image_backend preference

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)

skills/baoyu-slide-deck/references/config/preferences-schema.md

@@ -12,6 +12,7 @@ style: blueprint              # Preset name OR "custom"
 audience: general             # beginners | intermediate | experts | executives | general
 language: auto                # auto | en | zh | ja | etc.
 review: true                  # true = review outline before generation
+preferred_image_backend: auto # auto | ask | <backend-id>
 
 ## Custom Dimensions (only when style: custom)
 dimensions:
@@ -40,6 +41,7 @@ custom_styles:
 | `audience` | string | `general` | Default target audience |
 | `language` | string | `auto` | Output language (auto = detect from input) |
 | `review` | boolean | `true` | Show outline review before generation |
+| `preferred_image_backend` | string | `auto` | Image backend selection. `auto` = prefer runtime-native tool, fall back to the only installed backend, ask if multiple non-native are present. `ask` = always confirm on every run. `<backend-id>` (e.g., `codex-imagegen`, `baoyu-imagine`, `image_generate`) = pin this backend when available; fall back to `auto` when it isn't. Absent = `auto`. Resolution logic is documented in `SKILL.md`'s `## Image Generation Tools` section. |
 
 ### Custom Dimensions