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.
150 findings fixed: linter autofixes for whitespace style (wsl_v5,
nlreturn, noinlineerr), named returns removed, magic numbers replaced
with named constants, static errNotRegular error, explicit Close/Parse
error handling, unused cobra params renamed to _, and runReport/
runTrees/hashPass split into helpers to satisfy cyclop, gocognit, and
funlen. Behavior verified unchanged against the README smoke test.
