Add mcproxyctl CLI for gRPC admin API

Introduces a new command-line tool for managing mc-proxy via the gRPC
admin API over Unix socket. Commands include route and firewall rule
CRUD operations, health checks, and status queries.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/mcproxyctl/main.go

@@ -0,0 +1,11 @@
+package main
+
+import (
+	"os"
+)
+
+func main() {
+	if err := rootCmd().Execute(); err != nil {
+		os.Exit(1)
+	}
+}