repo: install styler and credo

Run `mix credo` once through, and create a simple justfile command to
check the code.

.formatter.exs

···
-# Used by "mix format"
 [
-  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"],
+  plugins: [Styler]
 ]

justfile

···
+check:
+    mix format
+    mix credo
+
+setup:
+    mix deps.get

mix.exs

···
   # Run "mix help deps" to learn about dependencies.
   defp deps do
     [
-      # {:dep_from_hexpm, "~> 0.3.0"},
-      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+      {:styler, "~> 1.9", only: [:dev, :test], runtime: false},
+      {:credo, "~> 1.7", only: [:dev, :test], runtime: false}
     ]
   end
 end

mix.lock

···
+%{
+  "bunt": {:hex, :bunt, "1.0.0", "081c2c665f086849e6d57900292b3a161727ab40431219529f13c4ddcf3e7a44", [:mix], [], "hexpm", "dc5f86aa08a5f6fa6b8096f0735c4e76d54ae5c9fa2c143e5a1fc7c1cd9bb6b5"},
+  "credo": {:hex, :credo, "1.7.13", "126a0697df6b7b71cd18c81bc92335297839a806b6f62b61d417500d1070ff4e", [:mix], [{:bunt, "~> 0.2.1 or ~> 1.0", [hex: :bunt, repo: "hexpm", optional: false]}, {:file_system, "~> 0.2 or ~> 1.0", [hex: :file_system, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "47641e6d2bbff1e241e87695b29f617f1a8f912adea34296fb10ecc3d7e9e84f"},
+  "file_system": {:hex, :file_system, "1.1.1", "31864f4685b0148f25bd3fbef2b1228457c0c89024ad67f7a81a3ffbc0bbad3a", [:mix], [], "hexpm", "7a15ff97dfe526aeefb090a7a9d3d03aa907e100e262a0f8f7746b78f8f87a5d"},
+  "jason": {:hex, :jason, "1.4.4", "b9226785a9aa77b6857ca22832cffa5d5011a667207eb2a0ad56adb5db443b8a", [:mix], [{:decimal, "~> 1.0 or ~> 2.0", [hex: :decimal, repo: "hexpm", optional: true]}], "hexpm", "c5eb0cab91f094599f94d55bc63409236a8ec69a21a67814529e8d5f6cc90b3b"},
+  "styler": {:hex, :styler, "1.9.1", "e30f0e909c02c686c75e47c07a76986483525eeb23c4d136f00dfa1c25fc6499", [:mix], [], "hexpm", "f583bedd92515245801f9ad504766255a27ecd5714fc4f1fd607de0eb951e1cf"},
+}

test/aoc2025_test.exs

···
 defmodule Aoc2025Test do
   use ExUnit.Case
+
   doctest Aoc2025
 
   test "greets the world" do