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

@@ -429,6 +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