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/test

@@ -0,0 +1,19 @@
+#!/bin/sh
+# script/test: run the test suite. Testing is a phase of the Dockerfile
+# and this builds that phase alone, on the same terms as script/lint:
+# --target because a phase that is not the last stage is built only when
+# named, --no-cache because a cached test layer is a test that did not
+# run, and a tag so each build replaces the previous image.
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    docker build --no-cache \
+        --target test \
+        -t "$("$SCRIPT_DIR/projectname")-test" .
+}
+
+main "$@"