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>

test/simpleshop_theme/theme/preview_data_test.exs

@@ -8,7 +8,7 @@ defmodule SimpleshopTheme.Theme.PreviewDataTest do
       products = PreviewData.products()
 
       assert is_list(products)
-      assert length(products) > 0
+      assert products != []
     end
 
     test "each product has required fields" do
@@ -62,7 +62,7 @@ defmodule SimpleshopTheme.Theme.PreviewDataTest do
       products = PreviewData.products()
       on_sale_products = Enum.filter(products, & &1.on_sale)
 
-      assert length(on_sale_products) > 0
+      assert on_sale_products != []
     end
 
     test "on-sale products have compare_at_price" do
@@ -80,7 +80,7 @@ defmodule SimpleshopTheme.Theme.PreviewDataTest do
       cart_items = PreviewData.cart_items()
 
       assert is_list(cart_items)
-      assert length(cart_items) > 0
+      assert cart_items != []
     end
 
     test "each cart item has required fields" do
@@ -120,7 +120,7 @@ defmodule SimpleshopTheme.Theme.PreviewDataTest do
       testimonials = PreviewData.testimonials()
 
       assert is_list(testimonials)
-      assert length(testimonials) > 0
+      assert testimonials != []
     end
 
     test "each testimonial has required fields" do
@@ -160,7 +160,7 @@ defmodule SimpleshopTheme.Theme.PreviewDataTest do
       categories = PreviewData.categories()
 
       assert is_list(categories)
-      assert length(categories) > 0
+      assert categories != []
     end
 
     test "each category has required fields" do