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

One command fetches the policy files a repo vendors from `prompts`, keeps the repo's own entries, commits them by name and pushes. A line the canonical file has never held stops the script before anything is written, as does a failed fetch. The canonical `.dockerignore` gains the closing comment the script needs. `test/update-policy.sh` runs in the `test` phase of `Dockerfile`.

Judgement call: `.gitea/workflows/check.yml` and `.prettierrc` are replaced whole.
Limitation: a repo with its own `.golangci.yml` settings, such as `sneak/homoicon`, stops until those are dealt with.

Model: opus-5-5

Makefile

@@ -1,4 +1,4 @@
-.PHONY: bootstrap setup test lint fmt fmt-check check docker hooks
+.PHONY: bootstrap setup test lint fmt fmt-check check docker hooks update-policy
 
 # Makefile targets are thin shims; the implementations live in script/
 # per the scripts-to-rule-them-all pattern (see the Entrypoints section
@@ -30,3 +30,6 @@ docker:
 
 hooks:
 	@script/install-precommit
+
+update-policy:
+	@script/update-policy