go styleguide: require Stringer for custom string wrapper types

Custom string-based types (e.g. ImageID, ContainerID) must implement
fmt.Stringer. Use .String() at SDK/library boundaries instead of
string(v) for explicit, grep-able conversions.

prompts/CODE_STYLEGUIDE_GO.md

@@ -423,6 +423,29 @@ last_modified: 2026-02-22
     [github.com/bcicen/go-units](https://github.com/bcicen/go-units) for
     temperatures (and others). The type system is your friend, use it.
 
+1.  When defining custom string-based wrapper types (e.g. `type ImageID string`,
+    `type ContainerID string`), always implement the `fmt.Stringer` interface.
+    At SDK and library boundaries, use `.String()` instead of `string(v)` to
+    convert the value to a plain string. This makes the conversion explicit,
+    grep-able, and ensures a consistent pattern across the codebase.
+
+    Example:
+
+    ```go
+    type ImageID string
+
+    func (id ImageID) String() string {
+        return string(id)
+    }
+
+    // At SDK/library boundaries:
+    // Good:
+    client.PullImage(ctx, imageID.String())
+
+    // Bad:
+    client.PullImage(ctx, string(imageID))
+    ```
+
 1.  Once you have a working program, run `go mod tidy` to clean up your `go.mod`
     and `go.sum` files. Tag a v0.0.1 or v1.0.0. Push your `main` branch and
     tag(s). Subsequent work should happen on branches so that `main` is "always