Re-vendor the canonical files from sneak/prompts at e3f79c4 (closes #95)

Every vendored file, REPO_POLICIES.md and every model script is the
copy at sneak/prompts e3f79c4, with this repository's own entries kept
after the canonical content. Lint and test are Dockerfile phases that
write no image; make test runs the suite under the race detector as
nobody. Every build in script/ passes a new CACHEBUST value, which every
stage declares first, so every RUN step runs again; the steps that
compile keep Go's build cache, and both go test runs pass -count=1.
Dockerfile.lint, script/verify-lint-image-pin and make test-race are
gone, prettier runs on the host, and .claude/settings.json is deleted.

Deviation: the set comes from e3f79c4 on next rather than dd4027b.
Judgement call: the test phase mounts the cache at nobody's
/tmp/.cache/go-build, owned by nobody.

Model: opus-5-5

script/fmt

@@ -1,22 +1,42 @@
 #!/bin/sh
-# script/fmt: format all files (writes): the Go sources with gofmt, the
-# Markdown with prettier. prettier is never installed on the host: it
-# runs from the Dockerfile's prettier stage with the repository mounted,
-# as the calling user so the files it rewrites keep their owner. The tag
-# makes each build replace the previous image instead of leaving another
-# one behind.
+# script/fmt: format all files (writes).
 set -eu
 
-SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
-ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+# Must match the pin in script/bootstrap.
+NODE_VERSION="22.17.0"
+
+# True when the node on PATH has the pinned major version: the same
+# function as in script/bootstrap, which uses it to decide where it
+# installs yarn.
+node_is_pinned_major() {
+    if ! command -v node >/dev/null 2>&1; then return 1; fi
+    installed="$(node --version)"
+    installed="${installed#v}"
+    [ "${installed%%.*}" = "${NODE_VERSION%%.*}" ]
+}
+
+# Run the yarn script/bootstrap installed: the yarn on PATH when the
+# node on PATH has the pinned major version, and otherwise yarn under
+# the pinned node in nvm, which bootstrap leaves off the PATH of the
+# shell that called it. nvm is a bash script, hence the subshell.
+run_yarn() {
+    if node_is_pinned_major && command -v yarn >/dev/null 2>&1; then
+        exec yarn "$@"
+    fi
+    if node_is_pinned_major || [ ! -s "$HOME/.nvm/nvm.sh" ]; then
+        echo "fmt: no yarn; run script/bootstrap first" >&2
+        exit 1
+    fi
+    exec bash -c '. "$HOME/.nvm/nvm.sh" && nvm use "$1" >/dev/null &&
+        shift && exec yarn "$@"' bash "$NODE_VERSION" "$@"
+}
 
 main() {
     cd "$ROOT"
     gofmt -s -w .
-    image="$("$SCRIPT_DIR/projectname")-prettier"
-    docker build -q --target prettier -t "$image" . >/dev/null
-    docker run --rm --user "$(id -u):$(id -g)" -v "$ROOT:/src" "$image" \
-        prettier --write '**/*.md' --tab-width 4 --prose-wrap always
+    run_yarn run prettier --write '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"