refactor(update): dedupe package manager detection

src/infra/detect-package-manager.ts

@@ -0,0 +1,29 @@
+import fs from "node:fs/promises";
+import path from "node:path";
+
+export type DetectedPackageManager = "pnpm" | "bun" | "npm";
+
+export async function detectPackageManager(root: string): Promise<DetectedPackageManager | null> {
+  try {
+    const raw = await fs.readFile(path.join(root, "package.json"), "utf-8");
+    const parsed = JSON.parse(raw) as { packageManager?: string };
+    const pm = parsed?.packageManager?.split("@")[0]?.trim();
+    if (pm === "pnpm" || pm === "bun" || pm === "npm") {
+      return pm;
+    }
+  } catch {
+    // ignore
+  }
+
+  const files = await fs.readdir(root).catch((): string[] => []);
+  if (files.includes("pnpm-lock.yaml")) {
+    return "pnpm";
+  }
+  if (files.includes("bun.lockb")) {
+    return "bun";
+  }
+  if (files.includes("package-lock.json")) {
+    return "npm";
+  }
+  return null;
+}