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

@@ -423,35 +423,35 @@ 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.  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
-    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.
+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
 
-    // String implements fmt.Stringer.
-    func (id ImageID) String() string { return string(id) }
+    func (id ImageID) String() string {
+        return string(id)
+    }
 
-    // Right:
-    client.RemoveImage(ctx, imageID.String())
+    // At SDK/library boundaries:
+    // Good:
+    client.PullImage(ctx, imageID.String())
 
-    // Wrong:
-    client.RemoveImage(ctx, string(imageID))
+    // 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
+    releasable". "Releasable" in this context means that it builds and functions
+    as expected, and that all tests and linting passes.
+
 # Other Golang Tips and Best Practices (Optional)
 
 1. For any internet-facing http server, set appropriate timeouts and limits to