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; this repo's own entries follow the
canonical text in .dockerignore, .gitignore and .editorconfig. 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.

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,