Add script/update-policy to re-vendor policy files (closes #162)

One command fetches the vendored files from prompts at a commit, keeps a repo's own entries, commits and pushes. The canonical .dockerignore gains the closing comment the script needs.

Model: opus-5-5

prompts/REPO_POLICIES.md

@@ -1,6 +1,6 @@
 ---
 title: Repository Policies
-last_modified: 2026-11-07
+last_modified: 2026-11-09
 ---
 
 This document covers repository structure, tooling, and workflow standards. Code
@@ -109,6 +109,7 @@ style conventions are in separate documents:
     - `script/fmt-check`
     - `script/check`
     - `script/docker`
+    - `script/update-policy`
 
     The scripts must be POSIX sh (`#!/bin/sh`, `set -eu`, no bashisms) so they
     run in minimal containers; alpine images, for example, have no bash. Locate
@@ -927,6 +928,17 @@ style conventions are in separate documents:
 - Make all changes on a feature branch. You can do whatever you want on a
   feature branch.
 
+- A repo updates its policy files with one command: `make update-policy`. It
+  runs `script/update-policy`, which fetches each vendored file the repo carries
+  from the `prompts` repo, commits them and pushes. It uses the head of `main`,
+  or the commit given by a full hash: `script/update-policy <hash>`. The list of
+  vendored files is at the top of that script. The script replaces each file and
+  keeps only the repo's own entries:
+    - in `.gitignore`, `.editorconfig` and `.dockerignore`: the lines below the
+      comment that ends the canonical file
+    - in `.golangci.yml`: the entries the repo added to the `deny` list of the
+      `test-support` depguard rule
+
 - `.golangci.yml` is standardized. The vendored copy in a consuming repo must
   _NEVER_ be modified by an agent. Fetch it from
   `https://git.eeqj.de/sneak/prompts/raw/branch/main/.golangci.yml` and keep it
@@ -1152,7 +1164,7 @@ style conventions are in separate documents:
     - `Makefile`
     - `script/` entrypoints (`bootstrap`, `setup`, `projectname`, `test`,
       `lint`, `fmt`, `fmt-check`, `check`, `docker`, `cibuild`, `precommit`,
-      `install-precommit`)
+      `install-precommit`, `update-policy`)
     - `Dockerfile`, `.dockerignore`
     - `.gitea/workflows/check.yml`
     - Go: `go.mod`, `go.sum`, `.golangci.yml`