style(go): add Stringer rule for custom string types

Custom string types should implement fmt.Stringer and use .String()
instead of string() casts when passing to library code.

prompts/CODE_STYLEGUIDE_GO.md

@@ -229,29 +229,6 @@ 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.
 
@@ -452,6 +429,29 @@ last_modified: 2026-02-22
     releasable". "Releasable" in this context means that it builds and functions
     as expected, and that all tests and linting passes.
 
+1.  Custom string types (e.g. `type ImageID string`) should implement the
+    `fmt.Stringer` interface by adding a `String() string` method. When passing
+    these types to library code or standard library functions that expect a
+    `string`, use the `.String()` method instead of a `string()` cast. This
+    keeps the code consistent with Go's interface conventions and makes it
+    easier to change the underlying representation later without updating every
+    call site.
+
+    Example:
+
+    ```go
+    type ImageID string
+
+    // String implements fmt.Stringer.
+    func (id ImageID) String() string { return string(id) }
+
+    // Right:
+    client.RemoveImage(ctx, imageID.String())
+
+    // Wrong:
+    client.RemoveImage(ctx, string(imageID))
+    ```
+
 # Other Golang Tips and Best Practices (Optional)
 
 1. For any internet-facing http server, set appropriate timeouts and limits to