refactor: remove proxy_id concept entirely

Not really needed and just adds extra fluff.

src/remote/endpoint.zig

@@ -165,7 +165,7 @@ const Endpoint = struct {
                 if (self.proxies.get(e.id)) |p|
                     p.send(.{ "exit", e.reason }) catch {};
             },
-            .proxy_id, .link, .transport_error => return tp.exit_error(error.UnexpectedMessage, null),
+            .link, .transport_error => return tp.exit_error(error.UnexpectedMessage, null),
         }
     }