Initial implementation of portainer-cli

Ruby CLI tool for interacting with the Portainer API. Supports listing
endpoints/stacks/containers/volumes/networks, creating stacks and containers,
and opening an interactive exec session in a container via WebSocket.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bin/portainer-cli

@@ -0,0 +1,7 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+$LOAD_PATH.unshift(File.expand_path('../lib', File.dirname(File.realpath(__FILE__))))
+
+require 'portainer_cli'
+PortainerCli::CLI.run(ARGV)