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/fmt-check

@@ -8,15 +8,25 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 # Must match the pin in script/bootstrap.
 NODE_VERSION="22.17.0"
 
-# script/bootstrap installs node and yarn under nvm and leaves neither
-# on the PATH of the shell that called it, so resolve the pinned
-# toolchain here the way bootstrap's own install step does. nvm is a
-# bash script, hence the subshell.
+# 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 command -v yarn >/dev/null 2>&1; then
+    if node_is_pinned_major && command -v yarn >/dev/null 2>&1; then
         exec yarn "$@"
     fi
-    if [ ! -s "$HOME/.nvm/nvm.sh" ]; then
+    if node_is_pinned_major || [ ! -s "$HOME/.nvm/nvm.sh" ]; then
         echo "fmt-check: no yarn; run script/bootstrap first" >&2
         exit 1
     fi