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

prompts/CODE_STYLEGUIDE_GO.md

@@ -238,6 +238,20 @@ last_modified: 2026-02-22
     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.