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)

docs/image-generation-tools.md

@@ -4,13 +4,29 @@ Skills in this repo are loaded by multiple agent runtimes (Claude Code, Codex, H
 
 ## The Rule
 
-When a skill needs to render an image:
+When a 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 the skill's `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.
 
-No explicit priority between runtime-native tools and repo skills — treat them equivalently and let the user decide when there's a choice. No persisted preference mechanism; the question is cheap, and the rule is stateless.
+Setting `preferred_image_backend: ask` forces the step-3 prompt every run regardless of available backends.
+
+## The Preference Field
+
+Each image-consuming skill's `EXTEND.md` carries a single `preferred_image_backend` field:
+
+| Value | Meaning |
+|---|---|
+| `auto` (default) | Apply the auto-select rule — runtime-native preferred, fall back to only installed backend, ask if multiple non-native. |
+| `ask` | Always confirm the backend on every run, even when a runtime-native tool exists. |
+| `<backend-id>` (e.g., `codex-imagegen`, `baoyu-imagine`, `image_generate`) | Pin this backend when available; fall back to `auto` if it isn't. |
+
+The field is **absent-equals-auto**: older `EXTEND.md` files without this field behave exactly as if `preferred_image_backend: auto` were set. No schema version bump is needed to introduce it.
 
 ## Prompt File Requirement (hard)
 
@@ -20,6 +36,8 @@ Regardless of which backend is chosen, every skill that renders images MUST writ
 
 Each `SKILL.md` that renders images includes **exactly one** `## Image Generation Tools` section (near the top, after `## User Input Tools` and before the main workflow) that **inlines** this rule. Skills are self-contained and cannot link to `docs/` — each skill folder must ship the rule inside its own `SKILL.md`. See [CLAUDE.md → Skill Self-Containment](../CLAUDE.md).
 
+Each skill's `references/config/preferences-schema.md` (and its `EXTEND.md` template in `first-time-setup.md`) lists `preferred_image_backend` alongside other preference fields. First-time setup does NOT ask the user about the backend — `auto` is set silently. Users who want to pin a specific backend edit `EXTEND.md` later, and each skill's `## Changing Preferences` section documents the common one-line edits.
+
 Concrete tool names (`imagegen`, `image_generate`, `baoyu-imagine`) in this document and in SKILL.md are **examples** — agents in other runtimes apply the rule above and substitute the local equivalent. Skill-specific parameters for these backends are illustrative; runtimes without those knobs can omit them.
 
 ## Backend Skills Are Exempt