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/cibuild

@@ -1,8 +1,10 @@
 #!/bin/sh
-# script/cibuild: run the CI build; the Gitea workflow runs this on push.
-# It builds the image with the same command as script/docker. --no-cache
-# because the checks the final stage depends on are RUN steps, and a
-# cached one is a check that did not run.
+# script/cibuild: run the CI build. It bootstraps first: a CI runner
+# checks out and runs this and nothing else, and script/fmt-check runs
+# the formatter on the host, which a pristine checkout cannot do.
+# --no-cache for the same reason as script/docker: the gate phases the
+# final stage depends on are RUN steps, and a cached one is a check that
+# did not run.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -10,6 +12,8 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
+    "$SCRIPT_DIR/bootstrap"
+    "$SCRIPT_DIR/check"
     # Own line: a failing command substitution inside an argument does
     # not trip `set -e`, so the inline form degrades silently to an
     # empty constant. The VERSION build argument takes precedence over