fix(baoyu-article-illustrator): omit model field from batch tasks when unspecified

When --model is not provided, build-batch no longer writes a model field
into batch task entries; baoyu-imagine resolves the default from env or
EXTEND config instead of inheriting a hardcoded script default.

Adds tests covering both the omit-by-default and explicit-override paths.

skills/baoyu-article-illustrator/scripts/build-batch.test.ts

@@ -0,0 +1,85 @@
+import assert from "node:assert/strict";
+import fs from "node:fs/promises";
+import os from "node:os";
+import path from "node:path";
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
+import test from "node:test";
+
+const execFileAsync = promisify(execFile);
+const repoRoot = path.resolve(import.meta.dirname, "..", "..", "..");
+const scriptPath = path.join(repoRoot, "skills", "baoyu-article-illustrator", "scripts", "build-batch.ts");
+
+async function makeFixture(): Promise<{
+  root: string;
+  outlinePath: string;
+  promptsDir: string;
+  outputPath: string;
+}> {
+  const root = await fs.mkdtemp(path.join(os.tmpdir(), "baoyu-article-illustrator-build-batch-"));
+  const outlinePath = path.join(root, "outline.md");
+  const promptsDir = path.join(root, "prompts");
+  const outputPath = path.join(root, "batch.json");
+
+  await fs.mkdir(promptsDir, { recursive: true });
+  await fs.writeFile(
+    outlinePath,
+    `## Illustration 1
+**Position**: demo
+**Purpose**: demo
+**Visual Content**: demo
+**Filename**: 01-demo.png
+`,
+  );
+  await fs.writeFile(path.join(promptsDir, "01-demo.md"), "A demo prompt\n");
+
+  return { root, outlinePath, promptsDir, outputPath };
+}
+
+async function runBuildBatch(args: string[]): Promise<void> {
+  await execFileAsync(process.execPath, ["--import", "tsx", scriptPath, ...args], {
+    cwd: repoRoot,
+  });
+}
+
+test("build-batch omits default model so baoyu-imagine can resolve env or EXTEND defaults", async () => {
+  const fixture = await makeFixture();
+
+  await runBuildBatch([
+    "--outline",
+    fixture.outlinePath,
+    "--prompts",
+    fixture.promptsDir,
+    "--output",
+    fixture.outputPath,
+  ]);
+
+  const batch = JSON.parse(await fs.readFile(fixture.outputPath, "utf8")) as {
+    tasks: Array<Record<string, unknown>>;
+  };
+
+  assert.equal(batch.tasks.length, 1);
+  assert.equal(batch.tasks[0]?.provider, "replicate");
+  assert.equal(Object.hasOwn(batch.tasks[0]!, "model"), false);
+});
+
+test("build-batch preserves explicit model overrides", async () => {
+  const fixture = await makeFixture();
+
+  await runBuildBatch([
+    "--outline",
+    fixture.outlinePath,
+    "--prompts",
+    fixture.promptsDir,
+    "--output",
+    fixture.outputPath,
+    "--model",
+    "acme/custom-model",
+  ]);
+
+  const batch = JSON.parse(await fs.readFile(fixture.outputPath, "utf8")) as {
+    tasks: Array<Record<string, unknown>>;
+  };
+
+  assert.equal(batch.tasks[0]?.model, "acme/custom-model");
+});