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. make fmt also formats
Go, using a pinned Go that script/bootstrap installs. 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. 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{. It prints 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 now compiles the library; it pointed at a
command that does not exist.

Model: opus-5-5

script/check

@@ -0,0 +1,16 @@
+#!/bin/sh
+# script/check: run all checks (test, lint, fmt-check). Our own
+# extension to scripts-to-rule-them-all. test and lint are Docker
+# phases; fmt-check is native, because a formatter writes the working
+# tree. Must not modify any files.
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+
+main() {
+    "$SCRIPT_DIR/test"
+    "$SCRIPT_DIR/lint"
+    "$SCRIPT_DIR/fmt-check"
+}
+
+main "$@"