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/internal/genid/name.go

@@ -0,0 +1,12 @@
+// Copyright 2024 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
+
+const (
+	NoUnkeyedLiteral_goname  = "noUnkeyedLiteral"
+	NoUnkeyedLiteralA_goname = "XXX_NoUnkeyedLiteral"
+
+	BuilderSuffix_goname = "_builder"
+)