bring the repo up to repo standards (closes #1)

Adopt the standard tooling from sneak/prompts: the Makefile and script/
entrypoints, lint and test stages in one Dockerfile, the lint config, the
CI workflow and the formatter settings. make fmt also formats Go, with a
pinned Go from script/bootstrap. The module path becomes
sneak.berlin/go/dcf.

Fix the lint findings in pkg/dcf. DCFStore becomes Store and DCFObject
becomes Object. Three behaviours change. An error listing mount points is
now returned, where a typo used to drop it. Store.String() prints Store{
instead of DCFStore{, and a negative total as 0 B instead of a huge size.

Replace the placeholder test with real tests. Move the README into the
required sections. make build compiles the library; it pointed at a
command that does not exist.

Model: opus-5-5

Makefile

@@ -1,14 +1,35 @@
-default: test
+.PHONY: bootstrap setup test lint fmt fmt-check check build docker hooks
+
+# Makefile targets are thin shims; the implementations live in script/
+# per the scripts-to-rule-them-all pattern (see the Entrypoints section
+# of README.md).
+
+bootstrap:
+	@script/bootstrap
+
+setup:
+	@script/setup
 
 test:
-	go test -v ./...
+	@script/test
 
-run: build
-	DEBUG=1 ./dcfinfo
+lint:
+	@script/lint
+
+fmt:
+	@script/fmt
+
+fmt-check:
+	@script/fmt-check
+
+check:
+	@script/check
 
 build:
-	go env -w CGO_ENABLED="0"
-	cd cmd/dcfinfo && go build -o ../../dcfinfo
+	@script/build
 
-clean:
-	rm -f dcfinfo
+docker:
+	@script/docker
+
+hooks:
+	@script/install-precommit