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>

Gemfile

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+source 'https://rubygems.org'
+
+gem 'websocket', '~> 1.2'