build: unify the gate so root make check covers the backend (closes #16)

Root make check, and with it the pre-commit hook, now gates the Go
backend too. The backend's Makefile targets are shims over
backend/script/*; script/cibuild builds both images and is the
workflow's only build step. Root make test runs both halves within one
30-second timeout.

Root make lint runs golangci-lint only in Docker, by building the lint
stage of Dockerfile.backend without the cache; the .golangci.yml drift
check moved into backend/script/lint. script/bootstrap installs no
linter: it reuses a Go at least as new as backend/go.mod asks for,
otherwise installs the pinned, hash-verified release, linked into
~/.local/bin without replacing anything it did not create. With VERSION
unset or empty, the backend version falls back to git describe inside a
git checkout, then to dev.

Model: opus-5-5

backend/script/fmt-check

@@ -0,0 +1,18 @@
+#!/bin/sh
+# script/fmt-check: check Go formatting (read-only). Same scope as
+# script/fmt, but fails instead of writing.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    unformatted="$(gofmt -l .)"
+    if [ -n "$unformatted" ]; then
+        echo "Files not formatted:" >&2
+        echo "$unformatted" >&2
+        exit 1
+    fi
+}
+
+main "$@"