Plain make now builds sfdupes (previously the default was all =
check + build); make build remains as an alias, so the Dockerfile
and existing habits keep working. The sfdupes target is phony: go
build's own cache decides what to recompile.
test/lint/fmt/fmt-check/check/docker/hooks plus build, all, and clean.
make check no longer builds the binary (it must not modify files); the
test target uses the 30s-timeout conditional -v rerun pattern. Version
is injected via -ldflags and exposed as sfdupes --version.
