Merge pull request 'style(go): add rule against type-only packages (per upaas #126 review)' (#2) from clawbot/prompts:add-no-type-only-packages-rule into main

Reviewed-on: #2

prompts/CODE_STYLEGUIDE_GO.md

@@ -229,6 +229,29 @@ last_modified: 2026-02-22
 
 1.  Define your struct types near their constructors.
 
+1.  Do not create packages whose sole purpose is to hold type definitions.
+    Packages named `types`, `domain`, or `models` that contain only structs and
+    interfaces (with no behavior) are a code smell. Define types alongside the
+    code that uses them. Type-only packages force consuming packages into alias
+    imports and circular-dependency gymnastics, and indicate that the package
+    boundaries were drawn around nouns instead of responsibilities. If multiple
+    packages need the same type, put it in the package that owns the behavior,
+    or in a small, focused interface package — not in a grab-bag types package.
+
+1.  When defining custom string-based types (e.g. `type ImageID string`),
+    implement `fmt.Stringer`. Use `.String()` at SDK and library boundaries
+    instead of `string(v)`. This makes type conversions explicit, grep-able,
+    and consistent across the codebase. Example:
+
+    ```go
+    type ContainerID string
+
+    func (id ContainerID) String() string { return string(id) }
+
+    // At the Docker SDK boundary:
+    resp, err := c.docker.ContainerStart(ctx, id.String(), opts)
+    ```
+
 1.  Define your interface types near the functions that use them, or if you have
     multiple conformant types, put the interface(s) in their own file.