fix(build): 补齐 hash 工具并新增 AGENTS 规范

frontend/src/utils/hash.ts

@@ -0,0 +1,21 @@
+export async function sha256HexOfBlob(blob: Blob): Promise<string | null> {
+  if (!('crypto' in globalThis) || !globalThis.crypto?.subtle) {
+    return null;
+  }
+
+  try {
+    const buffer = await blob.arrayBuffer();
+    const digest = await globalThis.crypto.subtle.digest('SHA-256', buffer);
+    return bytesToHex(new Uint8Array(digest));
+  } catch {
+    return null;
+  }
+}
+
+function bytesToHex(bytes: Uint8Array): string {
+  let hex = '';
+  for (const byte of bytes) {
+    hex += byte.toString(16).padStart(2, '0');
+  }
+  return hex;
+}