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/cache.test.ts

@@ -0,0 +1,63 @@
+import { test, expect } from "bun:test";
+import { mkdtemp, writeFile, readFile, rm } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { cacheKey, lookupCache, storeCache, FileLock } from "./cache.ts";
+
+test("cacheKey is deterministic and order-independent for refs", () => {
+  const k1 = cacheKey("hello", "16:9", ["a.png", "b.png"]);
+  const k2 = cacheKey("hello", "16:9", ["b.png", "a.png"]);
+  expect(k1).toBe(k2);
+  const k3 = cacheKey("hello", "16:9", []);
+  expect(k3).not.toBe(k1);
+  const k4 = cacheKey("hello", "1:1", []);
+  expect(k4).not.toBe(k3);
+});
+
+test("lookupCache returns null on miss, path on hit", async () => {
+  const dir = await mkdtemp(path.join(tmpdir(), "cig-test-"));
+  try {
+    expect(await lookupCache(dir, "abc")).toBeNull();
+    const fake = path.join(dir, "abc.png");
+    await writeFile(fake, Buffer.alloc(2000));
+    expect(await lookupCache(dir, "abc")).toBe(fake);
+  } finally {
+    await rm(dir, { recursive: true, force: true });
+  }
+});
+
+test("storeCache copies source into cache", async () => {
+  const dir = await mkdtemp(path.join(tmpdir(), "cig-test-"));
+  const src = path.join(dir, "src.png");
+  try {
+    await writeFile(src, Buffer.from("xxxx".repeat(1000)));
+    await storeCache(dir, "key1", src);
+    const cached = await lookupCache(dir, "key1");
+    expect(cached).not.toBeNull();
+    const a = await readFile(src);
+    const b = await readFile(cached!);
+    expect(a.equals(b)).toBe(true);
+  } finally {
+    await rm(dir, { recursive: true, force: true });
+  }
+});
+
+test("FileLock prevents concurrent acquisition", async () => {
+  const dir = await mkdtemp(path.join(tmpdir(), "cig-lock-"));
+  try {
+    const lockPath = path.join(dir, "x.lock");
+    const lock1 = new FileLock(lockPath);
+    const lock2 = new FileLock(lockPath);
+    await lock1.acquire(1000);
+    let lock2Acquired = false;
+    const p = lock2.acquire(500).then(() => (lock2Acquired = true)).catch(() => {});
+    await new Promise((r) => setTimeout(r, 300));
+    expect(lock2Acquired).toBe(false);
+    await lock1.release();
+    await p;
+    expect(lock2Acquired).toBe(true);
+    await lock2.release();
+  } finally {
+    await rm(dir, { recursive: true, force: true });
+  }
+});