bring the repo up to org standards

Adopt the standard tooling: a Makefile of thin shims over a full
scripts-to-rule-them-all `script/` set, the canonical `.golangci.yml`
vendored byte-identical from `sneak/prompts`, docker-only linting via
`Dockerfile.lint`, a `Dockerfile` and Gitea workflow that gate every
push, and prettier/editorconfig/dockerignore config.

`make build` pointed at a `cmd/dcfinfo` that is not in the tree and
could never have succeeded; this repo is a library, so `build` is now
the compile check over every package.

Clear the 57 findings the canonical linter config reports on `pkg/dcf`.
Two were real: `findDCFMountPoints` checked a never-assigned `erro`
instead of the error from `findAllMountPoints`, discarding it, and
`privatePath` was computed twice so the first computation was dead.
Mountpoint selection otherwise behaves exactly as before; the defects
that survive are filed as issue #6, not fixed here.

Rename `DCFStore` to `Store` and `DCFObject` to `Object` (with its
`DCFStoreRoot` field to `StoreRoot`), which revive's stutter rule
requires and which the `fs.FS` rework in issue #4 will build on.

Replace the placeholder test with tests over the exported surface.
The filesystem walk stays uncovered: it is reachable only through
`GetDCFStores`, which needs real mounted media.

README keeps its content, reorganised into the required sections and
gaining Entrypoints.

(closes #1)

script/test

@@ -0,0 +1,21 @@
+#!/bin/sh
+# script/test: run the test suite. Quiet on success; on failure, rerun
+# with -v for full diagnostic output (and still fail — the first run
+# already proved the tests are broken).
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    # -count=1 disables the test result cache. Without it a repeat run on
+    # an unchanged tree reports every package `ok ... (cached)` and exits
+    # 0 having executed nothing — a gate that cannot fail.
+    go test -count=1 -timeout 90s -race -cover ./... || {
+        echo "--- Rerunning with -v for details ---"
+        go test -count=1 -timeout 90s -race -v ./...
+        exit 1
+    }
+}
+
+main "$@"