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>

mix.exs

@@ -11,7 +11,8 @@ defmodule SimpleshopTheme.MixProject do
       aliases: aliases(),
       deps: deps(),
       compilers: [:phoenix_live_view] ++ Mix.compilers(),
-      listeners: [Phoenix.CodeReloader]
+      listeners: [Phoenix.CodeReloader],
+      dialyzer: [ignore_warnings: ".dialyzer_ignore.exs"]
     ]
   end
 
@@ -27,7 +28,7 @@ defmodule SimpleshopTheme.MixProject do
 
   def cli do
     [
-      preferred_envs: [precommit: :test]
+      preferred_envs: [precommit: :test, ci: :test]
     ]
   end
 
@@ -73,7 +74,9 @@ defmodule SimpleshopTheme.MixProject do
       {:oban, "~> 2.18"},
       {:ex_money, "~> 5.0"},
       {:ex_money_sql, "~> 1.0"},
-      {:stripity_stripe, "~> 3.2"}
+      {:stripity_stripe, "~> 3.2"},
+      {:credo, "~> 1.7", only: [:dev, :test], runtime: false},
+      {:dialyxir, "~> 1.4", only: [:dev, :test], runtime: false}
     ]
   end
 
@@ -102,7 +105,15 @@ defmodule SimpleshopTheme.MixProject do
         "esbuild simpleshop_theme --minify",
         "phx.digest"
       ],
-      precommit: ["compile --warning-as-errors", "deps.unlock --unused", "format", "test"]
+      precommit: ["compile --warning-as-errors", "deps.unlock --unused", "format", "test"],
+      ci: [
+        "compile --warning-as-errors",
+        "deps.unlock --unused",
+        "format --check-formatted",
+        "credo",
+        "dialyzer",
+        "test"
+      ]
     ]
   end
 end