perf(directory-readme-injector): migrate sync FS to fs.promises

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/directory-readme-injector/finder.ts

@@ -1,4 +1,4 @@
-import { existsSync } from "node:fs";
+import { access } from "node:fs/promises";
 import { dirname, isAbsolute, join, resolve } from "node:path";
 
 import { README_FILENAME } from "./constants";
@@ -9,17 +9,19 @@ export function resolveFilePath(rootDirectory: string, path: string): string | n
   return resolve(rootDirectory, path);
 }
 
-export function findReadmeMdUp(input: {
+export async function findReadmeMdUp(input: {
   startDir: string;
   rootDir: string;
-}): string[] {
+}): Promise<string[]> {
   const found: string[] = [];
   let current = input.startDir;
 
   while (true) {
     const readmePath = join(current, README_FILENAME);
-    if (existsSync(readmePath)) {
+    try {
+      await access(readmePath);
       found.push(readmePath);
+    } catch {
     }
 
     if (current === input.rootDir) break;