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 canonical lint
config, the CI workflow and the formatter settings. Change the module
path to sneak.berlin/go/dcf.

Fix the lint findings in pkg/dcf in place. DCFStore becomes Store and
DCFObject becomes Object. One behaviour changes: an error listing mount
points is now returned, where a typo used to drop it.

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

Model: opus-5-5

script/build

@@ -0,0 +1,14 @@
+#!/bin/sh
+# script/build: compile every package. dcf is a library, so this checks
+# that it compiles and writes no binary. It needs Go on the host, and the
+# final stage of the Dockerfile runs it too.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    CGO_ENABLED=0 go build ./...
+}
+
+main "$@"