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/rulimit.go

@@ -0,0 +1,19 @@
+// Copyright 2021 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 linux || darwin || netbsd || openbsd
+// +build linux darwin netbsd openbsd
+
+package sqlite // import "modernc.org/sqlite"
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+func setMaxOpenFiles(n int64) error {
+	var rLimit unix.Rlimit
+	rLimit.Max = uint64(n)
+	rLimit.Cur = uint64(n)
+	return unix.Setrlimit(unix.RLIMIT_NOFILE, &rLimit)
+}