Add Nix flake for mciasctl and mciasgrpcctl

Vendor dependencies and expose control program binaries via
nix build. Uses nixpkgs-unstable for Go 1.26 support.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/golang.org/x/net/http2/config_go126.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 h2.StrictMaxConcurrentRequests
+}