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/logger.ts

@@ -0,0 +1,39 @@
+import { appendFile, mkdir } from "node:fs/promises";
+import path from "node:path";
+
+export interface LogEntry {
+  ts: string;
+  level: "info" | "warn" | "error";
+  event: string;
+  [k: string]: unknown;
+}
+
+export class JsonLogger {
+  constructor(private logFile: string | null, public verbose: boolean) {}
+
+  async log(level: LogEntry["level"], event: string, extra: Record<string, unknown> = {}): Promise<void> {
+    const entry: LogEntry = { ts: new Date().toISOString(), level, event, ...extra };
+    const line = JSON.stringify(entry);
+    if (this.verbose) process.stderr.write(`[${level}] ${event} ${jsonExtras(extra)}\n`);
+    if (this.logFile) {
+      await mkdir(path.dirname(this.logFile), { recursive: true });
+      await appendFile(this.logFile, line + "\n", "utf-8");
+    }
+  }
+
+  info(event: string, extra?: Record<string, unknown>) {
+    return this.log("info", event, extra);
+  }
+  warn(event: string, extra?: Record<string, unknown>) {
+    return this.log("warn", event, extra);
+  }
+  error(event: string, extra?: Record<string, unknown>) {
+    return this.log("error", event, extra);
+  }
+}
+
+function jsonExtras(extra: Record<string, unknown>): string {
+  const entries = Object.entries(extra);
+  if (entries.length === 0) return "";
+  return entries.map(([k, v]) => `${k}=${typeof v === "string" ? v : JSON.stringify(v)}`).join(" ");
+}