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/SKILL.md

@@ -27,14 +27,22 @@ 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-slide-[slug].md`) BEFORE invoking any backend. The file is the reproducibility record and lets you switch backends without regenerating prompts.
 
+Concrete tool names (`imagegen`, `image_generate`, `baoyu-imagine`) above are examples — substitute the local equivalents under the same rule.
+
 ## Language
 
 Respond in the user's language across questions, progress reports, error messages, and the completion summary. Keep technical tokens (style names, file paths, code) in English.
@@ -320,4 +328,14 @@ See `references/modification-guide.md` for full details.
 - For sensitive public figures, prefer stylized alternatives to avoid likeness issues.
 - Maintain visual consistency via the session ID when the backend supports it.
 
-Custom configurations via EXTEND.md. See Step 1.1 for paths and schema.
+## Changing Preferences
+
+EXTEND.md lives at the first matching path listed in Step 1.1. Two ways to change it:
+
+- **Edit directly** — open EXTEND.md and change fields. Full schema: `references/config/preferences-schema.md`.
+- **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.
+  - `preferred_style: blueprint`, `preferred_audience: experts`, `language: zh`.

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