Add Credo and pay down tech debt: dedup generation paths, narrow rescues

.credo.exs

@@ -0,0 +1,20 @@
+# Credo configuration. Runs on top of Credo's default check set.
+%{
+  configs: [
+    %{
+      name: "default",
+      files: %{
+        included: ["lib/", "test/"],
+        excluded: []
+      },
+      checks: %{
+        extra: [
+          # NIF entry points mirror the positional C ABI in c_src/llama_cpp_ex —
+          # their arity is fixed by the native signatures, not a style choice.
+          # The keyword-based wrappers around them keep the public API ergonomic.
+          {Credo.Check.Refactor.FunctionArity, files: %{excluded: ["lib/llama_cpp_ex/nif.ex"]}}
+        ]
+      }
+    }
+  ]
+}