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/modernc.org/sqlite/nodmesg.go

@@ -0,0 +1,12 @@
+// Copyright 2023 The Sqlite 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 !sqlite.dmesg
+// +build !sqlite.dmesg
+
+package sqlite // import "modernc.org/sqlite"
+
+const dmesgs = false
+
+func dmesg(s string, args ...interface{}) {}