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.

README.md

@@ -115,6 +115,22 @@ subdirectory. Each file contains one or more related prompts or policy
 documents. There is no build step or runtime component; the prompts are consumed
 by copying them into other projects or referencing them directly.
 
+## Template Repos
+
+These template repositories implement the policies defined in this repo and
+serve as starting points for new projects. They must be kept in sync when
+policies change.
+
+- **[template-app-go](https://git.eeqj.de/sneak/template-app-go)** — Go HTTP
+  server template (Uber fx, chi, SQLite, session auth, Prometheus metrics)
+- **[template-app-js](https://git.eeqj.de/sneak/template-app-js)** — JavaScript
+  SPA template (Vite, Tailwind CSS v4, nginx Docker deployment)
+- **[template-app-python](https://git.eeqj.de/sneak/template-app-python)** —
+  Python web application template (FastAPI, uvicorn, pytest, black, ruff)
+
+When updating policies in this repo, also update the template repos to match
+(Makefile targets, Dockerfile conventions, CI workflows, required files, etc.).
+
 ## TODO
 
 - Add more prompt templates for common development tasks

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