Add Nix flake for mcrctl

Vendor dependencies and expose mcrctl binary via nix build.

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

vendor/modernc.org/sqlite/rlimit.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 freebsd
+// +build freebsd
+
+package sqlite // import "modernc.org/sqlite"
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+func setMaxOpenFiles(n int64) error {
+	var rLimit unix.Rlimit
+	rLimit.Max = n
+	rLimit.Cur = n
+	return unix.Setrlimit(unix.RLIMIT_NOFILE, &rLimit)
+}