feat(baoyu-cover-image): wire SKILL.md to call codex-imagegen wrapper

Per review feedback on #158: a backend wrapper alone is not enough —
skills need explicit invocation guidance so the agent can find and
call it.

Updates skills/baoyu-cover-image/SKILL.md in two places:

1. ## Image Generation Tools — new bullet under the backend resolution
   list: 'Codex via codex exec (codex-imagegen)' with the exact command
   shape (--image / --prompt-file / --aspect / --ref / --cache-dir /
   --log-file), JSON output handling, and prerequisites.

2. ### Step 4: Generate Image, step 5 — added a concrete example
   command for the codex-imagegen branch alongside the existing generic
   instruction.

End-to-end verified: with EXTEND.md set to preferred_image_backend:
codex-imagegen, calling the command shape written in SKILL.md produces
a 1672x941 PNG in 83s (status: ok, attempts: 1, cached: false).
Output: /tmp/cover-e2e/cover.png. Structured log:
/tmp/cover-e2e/run.jsonl.

baoyu-cover-image is the validation skill; if reviewers want the same
guidance in baoyu-article-illustrator / baoyu-comic / baoyu-image-cards
/ baoyu-infographic / baoyu-slide-deck, happy to mirror the change.

skills/baoyu-cover-image/SKILL.md

@@ -29,6 +29,17 @@ When this skill needs to render an image, resolve the backend in this order:
 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):
    - **Codex (`imagegen`)** — first, inspect your available-skills / tool inventory. If a skill named `imagegen` is listed, you are running inside Codex and MUST use it: invoke via the `Skill` tool with `skill: "imagegen"`, passing the saved prompt file's content (plus output path and aspect ratio per Codex `imagegen`'s own args). Codex `imagegen` is the official raster backend in that runtime and outranks any non-native skill (e.g., `baoyu-imagine`) unless the user has explicitly pinned a different `preferred_image_backend`.
+   - **Codex via `codex exec` (`codex-imagegen`)** — if the current runtime does NOT expose a native `imagegen` skill but the `codex` CLI is on `PATH` and `codex login` is active (e.g., Claude Code with Codex CLI installed), invoke the `codex-imagegen` wrapper. Command shape:
+     ```bash
+     ./scripts/codex-imagegen.sh \
+       --image <absolute_output_path> \
+       --prompt-file <prompts/NN-cover-[slug].md> \
+       --aspect <ratio> \
+       [--ref <file>]... \
+       [--cache-dir ~/.cache/baoyu-codex-imagegen] \
+       [--log-file <jsonl_log_path>]
+     ```
+     Parse the single-line JSON on stdout. On `{"status":"ok",...}` proceed to Step 5. On `{"status":"error","error_kind":...}` report the `error_kind` to the user and (if retryable) ask whether to retry or fall back to another backend. The wrapper uses the user's Codex subscription — no `OPENAI_API_KEY` needed.
    - **Other runtime-native tools** — if the runtime exposes a different native image tool (e.g., Hermes `image_generate`), use it the same way.
    - 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.
@@ -214,7 +225,9 @@ Save to `prompts/cover.md`. Template: [references/workflow/prompt-template.md](r
 4. **Process references** from prompt frontmatter:
    - `direct` usage → pass via `--ref` (use ref-capable backend)
    - `style`/`palette` → extract traits, append to prompt
-5. **Generate**: Call the chosen backend with the prompt file, output path, aspect ratio
+5. **Generate**: Call the chosen backend with the prompt file, output path, aspect ratio.
+   - **`codex-imagegen`**: run `./scripts/codex-imagegen.sh --image <output> --prompt-file prompts/01-cover-[slug].md --aspect <ratio>` (add `--ref <file>` per reference, `--cache-dir ~/.cache/baoyu-codex-imagegen` to enable the idempotency cache). Read the stdout JSON; act on `status` and `error_kind`.
+   - **Codex `imagegen` (native)** or other runtime-native tools / `baoyu-imagine` skill: per the rule in `## Image Generation Tools` above.
 6. On failure: auto-retry once
 
 ### Step 5: Completion Report