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

@@ -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.  When defining custom string-based wrapper types (e.g. `type ImageID string`,
+1.  Once you have a working program, run `go mod tidy` to clean up your `go.mod`
-    `type ContainerID string`), always implement the `fmt.Stringer` interface.
+    and `go.sum` files. Tag a v0.0.1 or v1.0.0. Push your `main` branch and
-    At SDK and library boundaries, use `.String()` instead of `string(v)` to
+    tag(s). Subsequent work should happen on branches so that `main` is "always
-    convert the value to a plain string. This makes the conversion explicit,
+    releasable". "Releasable" in this context means that it builds and functions
-    grep-able, and ensures a consistent pattern across the codebase.
+    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
 
-    func (id ImageID) String() string {
+    // String implements fmt.Stringer.
-        return string(id)
+    func (id ImageID) String() string { return string(id) }
-    }
 
-    // At SDK/library boundaries:
+    // Right:
-    // Good:
+    client.RemoveImage(ctx, imageID.String())
-    client.PullImage(ctx, imageID.String())
 
-    // Bad:
+    // Wrong:
-    client.PullImage(ctx, string(imageID))
+    client.RemoveImage(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