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.

cmd/bsdaily/main.go

@@ -21,8 +21,8 @@ func main() {
 	var toFlag string
 
 	rootCmd := &cobra.Command{
-		Use:   "bsdaily",
-		Short: "Extract a single day's data from the latest daily snapshot",
+		Use:          "bsdaily",
+		Short:        "Extract a single day's data from the latest daily snapshot",
 		SilenceUsage: true,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			hasDate := dateFlag != ""