Add cp command to copy files out of containers, binary-safe

`portainer-cli cp <stack> <container>:<path> <dest>` goes through Docker's
archive API, so unlike `console --run "cat …"` there's no TTY in between and
binary files (database dumps) arrive intact. Works like `docker cp` for files
and directories; a dest of "-" writes a single file to stdout.

Stack and container lookup moves from Console into a shared Resolver module.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/portainer_cli/client.rb

@@ -32,6 +32,22 @@ module PortainerCli
       request(:delete, path)
     end
 
+    # GET a raw (non-JSON) body, yielding it in chunks as it arrives
+    def stream(path, params = {}, &block)
+      uri = http_uri(path)
+      uri.query = URI.encode_www_form(params) unless params.empty?
+
+      req = Net::HTTP::Get.new(uri)
+      @config.auth_header.each { |k, v| req[k] = v }
+
+      build_http(uri).request(req) do |resp|
+        code = resp.code.to_i
+        raise ApiError.new("HTTP #{code}: #{resp.body}", code) if code >= 400
+
+        resp.read_body(&block)
+      end
+    end
+
     # Returns the raw websocket URL (ws:// or wss://) for exec sessions
     def websocket_url(path, params = {})
       uri = ws_uri(path)