docs: record confirmed headless auth headers (Authorization + Arcade-User-ID)

.cursor/mcp.json.example

@@ -1,11 +1,11 @@
 {
-  "_comment": "Copy to .cursor/mcp.json (git-ignored) and fill slug + headers. Header names per LIVE-POC.md (confirmed in Task 1.1). Use your own user_id value.",
+  "_comment": "Copy to .cursor/mcp.json (git-ignored) and fill the gateway slug + your user_id. Headers confirmed from Arcade docs (see LIVE-POC.md).",
   "mcpServers": {
     "arcade-eval": {
       "url": "https://api.arcade.st.dev/mcp/<slug>",
       "headers": {
         "Authorization": "Bearer ${ARCADE_API_KEY}",
-        "<user-id-header>": "<your-user-id>"
+        "Arcade-User-ID": "<your-user-id>"
       }
     }
   }