Re-vendor the standard files from sneak/prompts dd4027b (closes #33)

REPO_POLICIES.md, .golangci.yml, the CI workflow and the scripts the
policy keeps identical across repositories are copies of the files at
that commit. .gitignore, .editorconfig and the new .dockerignore are the
canonical files followed by this repository's own entries. The lint
stage moves to the golangci-lint image the policy pins, in the same
commit as .golangci.yml. The format check leaves the lint stage and runs
on the host from script/check, which script/cibuild now runs after
script/bootstrap. The last Dockerfile stage runs script/bootstrap.
script/fmt runs goimports with go run at a pinned commit.

Model: opus-5-5

script/bootstrap

@@ -55,6 +55,7 @@ main() {
 
     if missing go; then pkg_install go golang go go; fi
     # golangci-lint is not installed: it runs only in docker (script/lint).
+    # Nor is goimports: script/fmt runs it with `go run` at a pinned commit.
 
     go mod download