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/bootstrap

@@ -1,13 +1,24 @@
 #!/bin/sh
 # script/bootstrap: install all dependencies needed to build and develop
-# this repo. Idempotent; assumes nothing is present (not git, make, or
-# go). Base tooling comes from nix, apt, brew, or apk (detected in that
-# order). golangci-lint and prettier are never installed: they run via
-# docker only (script/lint, script/fmt, script/fmt-check).
+# this repo. Idempotent: every install is guarded by a check so already
+# installed tools are skipped. Base tooling comes from nix, apt, brew,
+# or apk (detected in that order); assumes nothing is present. The
+# installed node is used only when its major version is the pinned one;
+# otherwise (node missing, or another major) the pinned version is
+# installed via nvm (installing nvm itself first, from a hash-verified
+# release archive, never curl | sh), and yarn and the packages are
+# installed under it.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
+# Pinned versions, 2026-07-06
+NODE_VERSION="22.17.0"
+NVM_VERSION="0.40.3"
+# sha256 of https://github.com/nvm-sh/nvm/archive/refs/tags/v0.40.3.tar.gz
+NVM_SHA256="5f4d6aaa04a177dc93c985e31dbc411ab6b8c6e1e21d8015dbc1372625fcd1d0"
+YARN_VERSION="1.22.22"
+
 PKGMGR=""
 SUDO=""
 APT_UPDATED=""
@@ -40,6 +51,7 @@ pkg_install() {
     case "$PKGMGR" in
         nix) nix-env -iA "nixpkgs.$1" ;;
         apt)
+            # Package lists may be empty (fresh images); refresh once per run.
             if [ -z "$APT_UPDATED" ]; then
                 $SUDO env DEBIAN_FRONTEND=noninteractive apt-get update
                 APT_UPDATED=1
@@ -55,24 +67,103 @@ missing() {
     ! command -v "$1" >/dev/null 2>&1
 }
 
+# verify_sha256 <file> <expected-hash>
+verify_sha256() {
+    if command -v sha256sum >/dev/null 2>&1; then
+        actual="$(sha256sum "$1" | cut -d' ' -f1)"
+    else
+        actual="$(shasum -a 256 "$1" | cut -d' ' -f1)"
+    fi
+    if [ "$actual" != "$2" ]; then
+        echo "bootstrap: sha256 mismatch for $1" >&2
+        echo "  expected: $2" >&2
+        echo "  actual:   $actual" >&2
+        exit 1
+    fi
+}
+
+# nvm is a bash script; run a command in a bash with nvm loaded
+nvm_sh() {
+    bash -c ". \"\$HOME/.nvm/nvm.sh\" && $*"
+}
+
+ensure_nvm() {
+    [ -s "$HOME/.nvm/nvm.sh" ] && return 0
+    # nvm prerequisites; nvm itself requires bash
+    if missing bash; then pkg_install bash bash bash bash; fi
+    if missing curl; then pkg_install curl curl curl curl; fi
+    if missing git; then pkg_install git git git git; fi
+    tmp="$(mktemp -d)"
+    curl -fsSL -o "$tmp/nvm.tar.gz" \
+        "https://github.com/nvm-sh/nvm/archive/refs/tags/v${NVM_VERSION}.tar.gz"
+    verify_sha256 "$tmp/nvm.tar.gz" "$NVM_SHA256"
+    mkdir -p "$HOME/.nvm"
+    tar -xzf "$tmp/nvm.tar.gz" -C "$HOME/.nvm" --strip-components=1
+    rm -rf "$tmp"
+}
+
+# True when the node on PATH has the pinned major version. Only the
+# major is compared: the Dockerfile stages start from a node 22 alpine
+# image whose exact version is not the pin, and nvm cannot install a
+# prebuilt node on alpine. Another major is not used: the pinned yarn 1
+# prints a deprecation warning under node 24. script/fmt and
+# script/fmt-check carry this function unchanged, to run the yarn
+# installed here.
+node_is_pinned_major() {
+    if ! command -v node >/dev/null 2>&1; then return 1; fi
+    installed="$(node --version)"
+    installed="${installed#v}"
+    [ "${installed%%.*}" = "${NODE_VERSION%%.*}" ]
+}
+
+ensure_node() {
+    if node_is_pinned_major; then
+        echo "bootstrap: using node $(node --version)"
+        return 0
+    fi
+    ensure_nvm
+    nvm_sh "nvm install $NODE_VERSION"
+    nvm_sh "nvm use $NODE_VERSION >/dev/null && \
+        echo \"bootstrap: using node \$(node --version)\""
+}
+
+ensure_yarn() {
+    if ! node_is_pinned_major; then
+        nvm_sh "nvm use $NODE_VERSION >/dev/null && corepack enable && \
+            corepack prepare yarn@$YARN_VERSION --activate"
+        return 0
+    fi
+    if ! missing yarn; then return 0; fi
+    if ! missing corepack; then
+        corepack enable
+        corepack prepare "yarn@$YARN_VERSION" --activate
+    else
+        npm install -g "yarn@$YARN_VERSION"
+    fi
+}
+
+install_js_deps() {
+    if node_is_pinned_major; then
+        yarn install --frozen-lockfile
+    else
+        nvm_sh "nvm use $NODE_VERSION >/dev/null && cd \"$ROOT\" && \
+            yarn install --frozen-lockfile"
+    fi
+}
+
 main() {
     cd "$ROOT"
 
-    # Deliberately unpinned, so presence is the whole check: go.mod
-    # governs the Go version, and reproducible builds run in the
-    # digest-pinned Docker images.
-    if missing git; then pkg_install git git git git; fi
     if missing make; then pkg_install gnumake make make make; fi
+    if missing git; then pkg_install git git git git; fi
+    # Go builds the binary and runs gofmt. Presence is the whole check:
+    # go.mod names the Go version, and the tests and the linter run in
+    # digest-pinned images.
     if missing go; then pkg_install go golang go go; fi
 
-    # Warn, do not fail: only the targets named below, and the
-    # pre-commit hook, need docker.
-    if missing docker; then
-        echo "bootstrap: WARNING: docker not found; make lint, make fmt," >&2
-        echo "bootstrap: make fmt-check, make check, make docker and" >&2
-        echo "bootstrap: make test-race require it." >&2
-    fi
-
+    ensure_node
+    ensure_yarn
+    install_js_deps
     go mod download
 
     echo "bootstrap complete"