styleguide: add rule against type-only packages

Types should be defined alongside the implementations that use them,
not in separate 'types' or 'domain' packages. This avoids awkward
import aliases and keeps types close to their usage.

prompts/CODE_STYLEGUIDE_GO.md

@@ -227,6 +227,14 @@ last_modified: 2026-02-22
      }
     ```
 
+1.  Do not create packages just to hold types (e.g. `types`, `domain`,
+    `models` that contain only type definitions). Define types alongside the
+    implementations that use them. A type-only package is a sign that the types
+    are in the wrong place — move them to the package that actually operates on
+    them. This also avoids awkward import aliases (e.g.
+    `domaintypes "myapp/internal/domain"`) which are a code smell indicating
+    poor package naming.
+
 1.  Define your struct types near their constructors.
 
 1.  Define your interface types near the functions that use them, or if you have