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/golang.org/x/sys/unix/ptrace_darwin.go

@@ -0,0 +1,11 @@
+// Copyright 2020 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.
+
+//go:build darwin && !ios
+
+package unix
+
+func ptrace(request int, pid int, addr uintptr, data uintptr) error {
+	return ptrace1(request, pid, addr, data)
+}