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/runtime/protoiface/legacy.go

@@ -0,0 +1,15 @@
+// Copyright 2018 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 protoiface
+
+type MessageV1 interface {
+	Reset()
+	String() string
+	ProtoMessage()
+}
+
+type ExtensionRangeV1 struct {
+	Start, End int32 // both inclusive
+}