fix: resolve lint error - ensure email is properly typed as string

src/commands/onboard-auth.credentials.ts

@@ -10,8 +10,9 @@ export async function writeOAuthCredentials(
   agentDir?: string,
 ): Promise<void> {
   // Write to resolved agent dir so gateway finds credentials on startup.
+  const emailStr = typeof creds.email === "string" ? creds.email : "default";
   upsertAuthProfile({
-    profileId: `${provider}:${creds.email ?? "default"}`,
+    profileId: `${provider}:${emailStr}`,
     credential: {
       type: "oauth",
       provider,