fix: unref keepalive timer to prevent proxy hanging on exit

The setInterval keepalive ping was keeping the proxy process alive
after copilot closed stdin. Adding .unref() lets Node exit cleanly
when there are no other active handles.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: BrainSlugs83

index.js

@@ -163,7 +163,7 @@ async function callServerWithRetry(path, body) {
 
 // --- Keepalive: ping server every 2 minutes to prevent idle shutdown ---
 const KEEPALIVE_MS = 2 * 60_000;
-setInterval(async () => {
+const keepaliveTimer = setInterval(async () => {
   try {
     const result = await ping();
     if (!result) {
@@ -173,6 +173,7 @@ setInterval(async () => {
     }
   } catch {}
 }, KEEPALIVE_MS);
+keepaliveTimer.unref(); // Don't block process exit for keepalive
 
 // --- Start: ensure server, then expose MCP tools ---