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/SKILL.md

@@ -23,11 +23,17 @@ Concrete `AskUserQuestion` references below are examples — substitute the loca
 
 ## Image Generation Tools
 
-When this skill needs to render an image:
+When this skill needs to render an image, resolve the backend in this order:
 
-- **Use whatever image-generation tool or skill is available** in the current runtime — e.g., Codex `imagegen`, Hermes `image_generate`, `baoyu-imagine`, or any equivalent the user has installed.
-- **If multiple are available**, ask the user **once** at the start which to use (batch with any other initial questions).
-- **If none are available**, tell the user and ask how to proceed.
+1. **Current-request override** — if the user names a specific backend in the current message, use it.
+2. **Saved preference** — if `EXTEND.md` sets `preferred_image_backend` to a backend available right now, use it.
+3. **Auto-select** (when the preference is `auto`, unset, or the pinned backend isn't available):
+   - If the current runtime exposes a native image tool (e.g., Codex `imagegen`, Hermes `image_generate`), use it. Runtime-native tools are preferred by default — agents that know their own tool inventory should surface the native one here.
+   - Otherwise, if exactly one non-native backend is installed (e.g., `baoyu-imagine`), use it.
+   - Otherwise (multiple non-native backends with no runtime-native tool), ask the user once — batch with any other initial questions.
+4. **If none are available**, tell the user and ask how to proceed.
+
+Setting `preferred_image_backend: ask` forces the step-3 prompt every run regardless of available backends. Users change the pinned backend via the `## Changing Preferences` section below.
 
 **Prompt file requirement (hard)**: write each image's full, final prompt to a standalone file under `prompts/` (naming: `NN-{type}-[slug].md`) BEFORE invoking any backend. The backend receives the prompt file (or its content); the file is the reproducibility record and lets you switch backends without regenerating prompts.
 
@@ -228,13 +234,18 @@ Files:
 - **Characters**: Simplified silhouettes; NO realistic humans
 - **Title**: Use exact title from user/source; never invent
 
-## Extension Support
+## Changing Preferences
 
-Custom configurations via EXTEND.md. See **Step 0** for paths.
+EXTEND.md lives at the path noted in **Step 0**. Three ways to change it:
 
-Supports: Watermark | Preferred dimensions | Default aspect/output | Quick mode | Custom palettes | Language
-
-Schema: [references/config/preferences-schema.md](references/config/preferences-schema.md)
+- **Edit directly** — open EXTEND.md and change fields. Full schema: [references/config/preferences-schema.md](references/config/preferences-schema.md).
+- **Reconfigure interactively** — delete EXTEND.md (or ask "reconfigure baoyu-cover-image preferences" / "重新配置"). The next run re-triggers first-time setup.
+- **Common one-line edits**:
+  - `preferred_image_backend: auto` — default; runtime-native tool wins, falls back to the only installed backend, asks only if multiple non-native are present.
+  - `preferred_image_backend: codex-imagegen` — pin to Codex's built-in.
+  - `preferred_image_backend: baoyu-imagine` — pin to the baoyu-imagine skill.
+  - `preferred_image_backend: ask` — confirm backend every run.
+  - `watermark.enabled: true`, `preferred_type`, `preferred_palette`, `preferred_rendering`, `default_aspect`, `quick_mode: true`, `language` — shift the auto-selection defaults and confirmation flow.
 
 ## References
 

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"