add CI pipeline with credo and dialyzer

mix ci alias: compile --warning-as-errors, format --check-formatted,
credo, dialyzer, test. Credo configured with sensible defaults.
Dialyzer ignore file for false positives (Stripe types, Mix tasks,
ExUnit internals). Credo fixes: map_join, filter consolidation,
nesting extraction, cond→if simplification.

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

lib/simpleshop_theme/clients/printify.ex

@@ -89,7 +89,7 @@ defmodule SimpleshopTheme.Clients.Printify do
   """
   def get_shop_id do
     case get_shops() do
-      {:ok, shops} when is_list(shops) and length(shops) > 0 ->
+      {:ok, shops} when is_list(shops) and shops != [] ->
         {:ok, hd(shops)["id"]}
 
       {:ok, []} ->