Auto-discover credentials, models, and agents on harness startup

Read mounted secret files (Claude OAuth, OpenCode auth.json) and env
vars on boot, register them as credentials, fetch available models
from provider APIs, and create default agent configs for each viable
runtime+provider+model combination.

apps/harness/next.config.js

@@ -1,6 +1,7 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
   output: "standalone",
+  instrumentationHook: true,
 };
 
 module.exports = nextConfig;