Re-vendor the canonical files from sneak/prompts at dd4027b (closes #171)

.dockerignore, .gitignore, .prettierignore and REPO_POLICIES.md are
copied from sneak/prompts at dd4027b; .editorconfig, check.yml and
.prettierrc already matched. This repository's build artifacts follow
the copied content in .gitignore and, anchored, in .dockerignore. The
scripts follow the new policy: check runs fmt-check, cibuild bootstraps
and runs check before the image build, fmt and fmt-check find the
pinned yarn under nvm, and the image's last stage marks /app safe for
git. The build-context test expects the new pattern forms.

Judgement call: script/precommit runs lint and fmt-check without the tests, so the README's red-phase test commit can still land; the policy allows that form only where local testing is impossible.
Not applicable: the issue's .golangci.yml entries; this TypeScript repository has none.

Model: opus-5-5

script/cibuild

@@ -1,8 +1,7 @@
 #!/bin/sh
-# script/cibuild: run the CI build. The image's last stage depends on the
-# lint and test phases, so this one build runs eslint, prettier and the
-# suite once each and then compiles. Unlike the template it does not run
-# script/check first, which would run lint and the tests a second time.
+# 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.
@@ -13,11 +12,12 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
+    "$SCRIPT_DIR/bootstrap"
+    "$SCRIPT_DIR/check"
     # Own line: a failing command substitution inside an argument does
     # not trip `set -e`, so the inline form degrades silently to an
-    # empty constant. The version resolved here goes in as the VERSION
-    # build arg, which takes precedence over what the build would derive
-    # from the .git in its context.
+    # empty constant. The VERSION build argument takes precedence over
+    # the version a build stage derives from the .git in the context.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
     docker build --no-cache \