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
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

script/build

@@ -0,0 +1,18 @@
+#!/bin/sh
+# script/build: compile every package. dcf is a library, so this checks
+# that it compiles and writes no binary. It needs the Go that
+# script/bootstrap installs, and the final stage of the Dockerfile runs
+# it too.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+# Where script/bootstrap installs Go; it cannot put it on our PATH.
+PATH="$HOME/.local/go/bin:$PATH"
+
+main() {
+    cd "$ROOT"
+    CGO_ENABLED=0 go build ./...
+}
+
+main "$@"