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.lock

@@ -0,0 +1,14 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    websocket (1.2.11)
+
+PLATFORMS
+  arm64-darwin-25
+  ruby
+
+DEPENDENCIES
+  websocket (~> 1.2)
+
+BUNDLED WITH
+   2.7.2