refactor(codex-imagegen): extract backend to packages/baoyu-codex-imagegen workspace

Move the codex-imagegen wrapper out of scripts/ into its own workspace
package alongside baoyu-md, baoyu-chrome-cdp, and baoyu-fetch. Drop the
bash shim — src/main.ts now carries a `#\!/usr/bin/env bun` shebang and
serves as the sole entrypoint. Add scripts/sync-codex-imagegen.sh so
skills/baoyu-image-gen/scripts/codex-imagegen/ can be regenerated from
packages/baoyu-codex-imagegen/src/ for skill self-containment. Update
CLAUDE.md, docs/codex-imagegen-backend.md, and the CI workflow paths
accordingly.

packages/baoyu-codex-imagegen/src/validator.ts

@@ -0,0 +1,55 @@
+import { stat, readdir } from "node:fs/promises";
+import { homedir } from "node:os";
+import path from "node:path";
+import { GenError } from "./types.ts";
+import type { ToolCall } from "./types.ts";
+
+const PNG_MAGIC = Buffer.from([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]);
+
+export function codexHome(): string {
+  return process.env.CODEX_HOME ?? path.join(homedir(), ".codex");
+}
+
+export async function verifyImageGenWasInvoked(threadId: string | null): Promise<{ ok: boolean; reason?: string }> {
+  if (!threadId) return { ok: false, reason: "no thread id" };
+  const dir = path.join(codexHome(), "generated_images", threadId);
+  try {
+    const entries = await readdir(dir);
+    const pngs = entries.filter((e) => e.toLowerCase().endsWith(".png"));
+    if (pngs.length === 0) return { ok: false, reason: `no PNG in ${dir}` };
+    return { ok: true };
+  } catch (e: any) {
+    return { ok: false, reason: `cannot read ${dir}: ${e?.code ?? e?.message}` };
+  }
+}
+
+export function findCpToTarget(toolCalls: ToolCall[], target: string): boolean {
+  return toolCalls.some(
+    (tc) =>
+      tc.tool === "shell" &&
+      typeof tc.command === "string" &&
+      (tc.command.includes(target) || tc.command.includes(path.basename(target))) &&
+      /\b(cp|mv|cat)\b/.test(tc.command) &&
+      tc.command.includes("generated_images"),
+  );
+}
+
+export async function verifyOutput(outputPath: string): Promise<{ bytes: number }> {
+  let s;
+  try {
+    s = await stat(outputPath);
+  } catch {
+    throw new GenError("output_missing", `Output file not created: ${outputPath}`);
+  }
+  if (s.size < 1000) {
+    throw new GenError("invalid_png", `Output file too small (${s.size} bytes)`);
+  }
+  const file = Bun.file(outputPath);
+  const head = new Uint8Array(await file.slice(0, 8).arrayBuffer());
+  for (let i = 0; i < PNG_MAGIC.length; i++) {
+    if (head[i] !== PNG_MAGIC[i]) {
+      throw new GenError("invalid_png", `Output is not a valid PNG (magic mismatch)`);
+    }
+  }
+  return { bytes: s.size };
+}