Add README, LICENSE, Makefile, Dockerfile, and CI

Add a detailed README, WTFPL LICENSE, and build/CI tooling modeled on
the vaultik repo (Makefile, multi-stage digest-pinned Dockerfile,
.gitea/workflows/check.yml). Bump Go to 1.26.4 and pin golangci-lint
to v2.12.2. gofmt existing sources so the new fmt-check gate passes.

internal/bsdaily/copy.go

@@ -69,4 +69,4 @@ func CopyFile(src, dst string) (err error) {
 		"elapsed", elapsed.Round(time.Millisecond),
 		"throughput_mbps", fmt.Sprintf("%.1f", throughputMBps))
 	return nil
-}
+}