Fix harness forbidden error: use internal token instead of host check

The localhost check using host header and x-forwarded-for was unreliable
in the standalone Next.js server which may inject forwarded headers
internally. Replace with a per-process random token shared between the
PTY server and the API route via env var.

apps/harness/pty-server.js

@@ -53,6 +53,7 @@ function attachPtyWebSocket(server) {
     try {
       const res = await fetch(
         `http://127.0.0.1:${port}/api/agents/${encodeURIComponent(agentId)}/env`,
+        { headers: { "x-internal-token": process.env.INTERNAL_API_TOKEN || "" } },
       );
       if (!res.ok) {
         const body = await res.json().catch(() => ({}));