Rewrite Makefile with required policy targets

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.

Makefile

@@ -2,16 +2,43 @@
 # toolchain.
 export CGO_ENABLED = 0
 
-.PHONY: all build check clean
+BINARY := sfdupes
+VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
+LDFLAGS := -X main.Version=$(VERSION)
 
-all: build
+.PHONY: all build test lint fmt fmt-check check docker hooks clean
+
+all: check build
 
 build:
-	go build
+	go build -ldflags "$(LDFLAGS)" -o $(BINARY)
 
-check: build
+test:
-	test -z "$$(gofmt -l .)"
+	@go test -timeout 30s -cover ./... || \
-	go vet ./...
+		{ echo "--- Rerunning with -v for details ---"; \
+		  go test -timeout 30s -v ./...; exit 1; }
+
+lint:
+	golangci-lint run --config .golangci.yml ./...
+
+fmt:
+	gofmt -s -w .
+
+fmt-check:
+	@files="$$(gofmt -l -s .)"; if [ -n "$$files" ]; then \
+		echo "gofmt: files not formatted:"; echo "$$files"; exit 1; fi
+
+check: test lint fmt-check
+
+docker:
+	docker build -t $(BINARY) .
+
+hooks:
+	@printf '#!/bin/sh\nset -e\n' > .git/hooks/pre-commit
+	@printf 'go mod tidy\ngo fmt ./...\n' >> .git/hooks/pre-commit
+	@printf 'git diff --exit-code -- go.mod go.sum || { echo "go mod tidy changed files; please stage and retry"; exit 1; }\n' >> .git/hooks/pre-commit
+	@printf 'make check\n' >> .git/hooks/pre-commit
+	@chmod +x .git/hooks/pre-commit
 
 clean:
-	rm -f sfdupes files.dat
+	rm -f $(BINARY) files.dat

main.go

@@ -27,11 +27,18 @@ const (
 	exitUsage = 2
 )
 
+// Version is the build version, injected at link time via -ldflags
+// (see the Makefile); "dev" for a plain go build.
+//
+//nolint:gochecknoglobals // written only by the linker
+var Version = "dev"
+
 func main() {
 	root := &cobra.Command{
-		Use:   "sfdupes",
+		Use:     "sfdupes",
-		Short: "Find candidate duplicate files by size and head/tail SHA-256",
+		Short:   "Find candidate duplicate files by size and head/tail SHA-256",
-		Args:  cobra.NoArgs,
+		Version: Version,
+		Args:    cobra.NoArgs,
 		Run: func(cmd *cobra.Command, _ []string) {
 			// A missing subcommand prints usage and exits 2.
 			_ = cmd.Usage()