Re-vendor the canonical files from sneak/prompts dd4027b (closes #159, closes #116)

Fetches .dockerignore, .editorconfig, the CI workflow, .gitignore,
.golangci.yml, .prettierignore, .prettierrc and REPO_POLICIES.md byte
for byte from sneak/prompts dd4027b, keeping this repo's anchored
host-built artifacts in .dockerignore; the new .golangci.yml disables
gomodguard. The Dockerfile gets a lint phase on golangci-lint v2.14.0
and a test phase on the Debian Go image; the build stage depends on
both and stamps the version as the policy shows. script/test and
script/lint build only their phase, and script/cibuild bootstraps and
runs script/check first. bin/tools goes: script/bootstrap installs
gofumpt and protoc-gen-go into bin/ with go install pinned to a commit,
and bin/.gitignore ignores what lands in bin/.

Model: opus-5-5

script/prettier

@@ -56,8 +56,7 @@ main() {
 
     # Markdown and JSON, repo-wide rather than root-only, so files in
     # subdirectories (docs/, once it exists) are covered too. Exclusions
-    # live in .prettierignore; REPO_POLICIES.md is excluded there because
-    # it is a verbatim copy of an upstream document.
+    # live in .prettierignore.
     #
     # --no-error-on-unmatched-pattern is deliberately NOT used: both
     # patterns always match at least one tracked file (README.md,