Make container name optional in console command

When omitted, tries: 1) same name as stack, 2) 'app'.
So 'portainer-cli console librarian' resolves to librarian-librarian-1,
and 'portainer-cli console myapp' falls back to myapp-app-1.

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

lib/portainer_cli/commands/console.rb

@@ -25,8 +25,8 @@ module PortainerCli
           o.on('--shell SHELL', 'Shell to use (default: /bin/sh)') { |v| opts[:shell] = v }
         end.parse!(args)
 
-        stack_name     = args.shift or error("Usage: portainer-cli console <stack> <container> [--shell /bin/bash]")
-        container_name = args.shift or error("Usage: portainer-cli console <stack> <container> [--shell /bin/bash]")
+        stack_name     = args.shift or error("Usage: portainer-cli console <stack> [container] [--shell /bin/bash]")
+        container_name = args.shift || stack_name
 
         stack       = resolve_stack(stack_name)
         endpoint_id = stack['EndpointId']
@@ -107,6 +107,18 @@ module PortainerCli
           error("Ambiguous container '#{name_or_id}'. Did you mean one of:\n#{list}")
         end
 
+        # 4. Try 'app' as a last resort when the implicit default didn't match
+        if name_or_id != 'app' && stack_name
+          app_candidates = containers.select do |c|
+            names_for.(c).any? { |n| n.match?(/\A#{Regexp.escape(stack_name)}-app-\d+\z/) }
+          end
+          if app_candidates.size == 1
+            matched_name = names_for.(app_candidates.first).first
+            $stderr.puts dim("Matched '#{name_or_id}' → #{matched_name} (fallback to 'app')")
+            return app_candidates.first['Id']
+          end
+        end
+
         available = pool.map { |c| "  #{c['Id'][0, 12]}  #{names_for.(c).first}" }.join("\n")
         error("Container '#{name_or_id}' not found. Available:\n#{available}")
       end