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/encoding/protojson/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 protojson marshals and unmarshals protocol buffer messages as JSON
+// format. It follows the guide at
+// https://protobuf.dev/programming-guides/proto3#json.
+//
+// This package produces a different output than the standard [encoding/json]
+// package, which does not operate correctly on protocol buffer messages.
+package protojson