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/net/http2/config_go125.go

@@ -0,0 +1,15 @@
+// Copyright 2025 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 !go1.26
+
+package http2
+
+import (
+	"net/http"
+)
+
+func http2ConfigStrictMaxConcurrentRequests(h2 *http.HTTP2Config) bool {
+	return false
+}