Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/google.golang.org/protobuf/internal/genid/doc.go

@@ -0,0 +1,11 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package genid contains constants for declarations in descriptor.proto
+// and the well-known types.
+package genid
+
+import "google.golang.org/protobuf/reflect/protoreflect"
+
+const GoogleProtobuf_package protoreflect.FullName = "google.protobuf"