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/lint

@@ -1,8 +1,13 @@
 #!/bin/sh
-# script/lint: run golangci-lint, in Docker only. Builds the lint stage of
-# the Dockerfile, whose build runs the linter, so a successful build is a
-# clean lint. --no-cache because a cached build runs no linter. The image
-# is removed afterwards, whatever the outcome.
+# script/lint: run the linter. Linting is a phase of the Dockerfile and
+# this builds that phase alone; the linter is never installed or run on
+# a developer host, where a shared result cache and a host-global lock
+# make its answer untrustworthy.
+#
+# The phase is not the last stage in the file, so it is built only when
+# --target names it. --no-cache because a cached lint layer is a lint
+# that did not run. The tag makes each build replace the previous image
+# instead of leaving a dangling one behind.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -10,11 +15,9 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    # Tagged per run, so concurrent runs never remove each other's image.
-    image="$("$SCRIPT_DIR/projectname")-lint:$$"
-    # A failed build leaves no image, so there is nothing to remove then.
-    trap 'docker image rm "$image" >/dev/null 2>&1 || true' EXIT INT TERM
-    docker build --no-cache --target lint -t "$image" .
+    docker build --no-cache \
+        --target lint \
+        -t "$("$SCRIPT_DIR/projectname")-lint" .
 }
 
 main "$@"