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/precommit

@@ -2,17 +2,17 @@
 # script/precommit: run by the git pre-commit hook; fails the commit if
 # checks fail. Our own extension to scripts-to-rule-them-all.
 #
-# Runs lint but deliberately NOT the tests, so the TDD red-phase commit
-# (failing tests, no implementation yet) can land. CI runs
-# script/cibuild, whose image build includes the test phase, and so
-# catches any branch that ships red. The lint phase includes the
-# prettier check, so a badly formatted tree still fails the commit.
+# Runs lint and fmt-check but deliberately NOT the tests, so the TDD
+# red-phase commit (failing tests, no implementation yet) can land. CI
+# runs script/cibuild, which runs the tests, and so catches any branch
+# that ships red.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
 
 main() {
     "$SCRIPT_DIR/lint"
+    "$SCRIPT_DIR/fmt-check"
 }
 
 main "$@"