Re-vendor the canonical files from sneak/prompts at e3f79c4 (closes #221)

The vendored dotfiles, REPO_POLICIES.md and the standard scripts are
fetched from sneak/prompts commit e3f79c4, with pixa's own entries kept
after the standard part. The scripts pass a new CACHEBUST build argument
in place of --no-cache and every Dockerfile stage declares it first, so
every RUN step reruns while Go's build cache, mounted on the steps that
compile, is kept. The lint phase runs golangci-lint v2.14.0, the test
phase runs on the Debian Go image, and the build stage takes the version
as the policy gives it. The two new lint findings are fixed in the code.

Model: opus-5-5

script/bootstrap

@@ -3,11 +3,13 @@
 # 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 (not git,
-# make, or go). Node is used directly if installed; otherwise it is
-# installed at a pinned version via nvm (installing nvm itself first,
-# from a hash-verified release archive, never curl | sh). The linter is
-# never installed on the host: golangci-lint runs only in the lint phase
-# of the Dockerfile (see script/lint).
+# make, or go). 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. The linter is never installed on
+# the host: golangci-lint runs only in the lint phase of the Dockerfile
+# (see script/lint).
 #
 #   script/bootstrap         git, make, Go, and Node, Yarn and the
 #                            prettier in yarn.lock for script/fmt and
@@ -110,31 +112,50 @@ ensure_nvm() {
     rm -rf "$tmp"
 }
 
+# True when the node on PATH has the pinned major version. Only the
+# major is compared, as REPO_POLICIES.md asks. 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 ! missing node; then return 0; fi
+    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@$YARN_VERSION" --activate
-    elif [ -s "$HOME/.nvm/nvm.sh" ]; then
-        nvm_sh "nvm use $NODE_VERSION >/dev/null && corepack enable && \
-            corepack prepare yarn@$YARN_VERSION --activate"
     else
         npm install -g "yarn@$YARN_VERSION"
     fi
 }
 
 install_js_deps() {
-    if missing yarn && [ -s "$HOME/.nvm/nvm.sh" ]; then
+    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"
-    else
-        yarn install --frozen-lockfile
     fi
 }