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-comic/SKILL.md

@@ -27,11 +27,17 @@ 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.
 
@@ -295,3 +301,16 @@ If EXTEND.md is not found, first-time setup is **blocking** — complete it befo
 - **Step 7.1 character sheet** - recommended for multi-page comics, optional for simple presets
 - **Step 7.2 character reference** - use `--ref` if sheet exists; compress/convert on failure; fall back to prompt-only
 - Watermark/language configured once in EXTEND.md
+
+## Changing Preferences
+
+EXTEND.md lives at `.baoyu-skills/baoyu-comic/EXTEND.md` (project) or `~/.baoyu-skills/baoyu-comic/EXTEND.md` (user). Three ways to change it:
+
+- **Edit directly** — open EXTEND.md and change fields. Full schema: `references/config/preferences-schema.md`.
+- **Reconfigure interactively** — delete EXTEND.md (or ask "reconfigure baoyu-comic 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_art`, `preferred_tone`, `preferred_layout`, `language` — shift the auto-selection defaults and cosmetic choices.

skills/baoyu-comic/references/config/first-time-setup.md

@@ -142,13 +142,13 @@ preferred_tone: [selected tone or null]
 preferred_layout: null
 preferred_aspect: null
 language: [selected or null]
+preferred_image_backend: auto
 character_presets: []
 ---
 ```
 
+`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: `config/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: `config/preferences-schema.md`.

skills/baoyu-comic/references/config/preferences-schema.md

@@ -23,6 +23,8 @@ preferred_aspect: null    # 3:4|4:3|16:9
 
 language: null            # zh|en|ja|ko|auto
 
+preferred_image_backend: auto  # auto|ask|<backend-id>
+
 character_presets:
   - name: my-characters
     roles:
@@ -46,6 +48,7 @@ character_presets:
 | `preferred_layout` | string | null | Layout preference or null |
 | `preferred_aspect` | string | null | Aspect ratio (3:4, 4:3, 16:9) |
 | `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. |
 | `character_presets` | array | [] | Preset character roles for styles like ohmsha |
 
 ## Art Style Options
@@ -122,6 +125,8 @@ preferred_aspect: "3:4"
 
 language: zh
 
+preferred_image_backend: codex-imagegen
+
 character_presets:
   - name: tech-tutorial
     roles: