Add Nix flake for mcrctl

Vendor dependencies and expose mcrctl binary via nix build.

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

vendor/google.golang.org/protobuf/proto/proto_reflect.go

@@ -0,0 +1,20 @@
+// 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.
+
+// The protoreflect build tag disables use of fast-path methods.
+//go:build protoreflect
+// +build protoreflect
+
+package proto
+
+import (
+	"google.golang.org/protobuf/reflect/protoreflect"
+	"google.golang.org/protobuf/runtime/protoiface"
+)
+
+const hasProtoMethods = false
+
+func protoMethods(m protoreflect.Message) *protoiface.Methods {
+	return nil
+}