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-cover-image/references/config/first-time-setup.md

@@ -188,15 +188,15 @@ default_aspect: [16:9/2.35:1/1:1/3:4]
 default_output_dir: [independent/same-dir/imgs-subdir]
 quick_mode: [true/false]
 language: null
+preferred_image_backend: auto
 custom_palettes: []
 ---
 ```
 
+`preferred_image_backend: auto` is the baked-in default — first-time setup does not ask about it. The `## Image Generation Tools` rule in SKILL.md then picks the runtime-native tool (Codex `imagegen`, Hermes `image_generate`, etc.) when available, and falls back to installed backends.
+
 ## Modifying Preferences Later
 
-Users can edit EXTEND.md directly or run setup again:
-- Delete EXTEND.md to trigger setup
-- Edit YAML frontmatter for quick changes
-- Full schema: `preferences-schema.md`
+See the `## Changing Preferences` section in `SKILL.md` for the canonical list of common edits (pin backend, change defaults, retrigger setup). Full schema: `preferences-schema.md`.
 
-**EXTEND.md Supports**: Watermark | Preferred type | Preferred palette | Preferred rendering | Preferred text | Preferred mood | Default aspect ratio | Default output directory | Quick mode | Custom palette definitions | Language preference
+**EXTEND.md Supports**: Watermark | Preferred type | Preferred palette | Preferred rendering | Preferred text | Preferred mood | Default aspect ratio | Default output directory | Quick mode | Image backend preference | Custom palette definitions | Language preference

skills/baoyu-cover-image/references/config/preferences-schema.md

@@ -32,6 +32,8 @@ quick_mode: false         # Skip confirmation when true
 
 language: null            # zh|en|ja|ko|auto (null = auto-detect)
 
+preferred_image_backend: auto  # auto|ask|<backend-id>
+
 custom_palettes:
   - name: my-palette
     description: "Palette description"
@@ -60,6 +62,7 @@ custom_palettes:
 | `default_aspect` | string | "2.35:1" | Default aspect ratio |
 | `quick_mode` | bool | false | Skip confirmation step |
 | `language` | string | null | Output language (null = auto-detect) |
+| `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_palettes` | array | [] | User-defined palettes |
 
 ## Type Options
@@ -187,6 +190,8 @@ quick_mode: true
 
 language: en
 
+preferred_image_backend: codex-imagegen
+
 custom_palettes:
   - name: corporate-tech
     description: "Professional B2B tech palette"