support targeting specific date

cmd/bsdaily/main.go

@@ -1,10 +1,13 @@
 package main
 
 import (
+	"fmt"
 	"log/slog"
 	"os"
+	"time"
 
 	"git.eeqj.de/sneak/bsdaily/internal/bsdaily"
+	"github.com/spf13/cobra"
 )
 
 func main() {
@@ -13,9 +16,33 @@ func main() {
 	}))
 	slog.SetDefault(logger)
 
-	if err := bsdaily.Run(); err != nil {
-		slog.Error("fatal error", "error", err)
+	var dateFlag string
+
+	rootCmd := &cobra.Command{
+		Use:   "bsdaily",
+		Short: "Extract a single day's data from the latest daily snapshot",
+		SilenceUsage: true,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			var targetDate *time.Time
+			if dateFlag != "" {
+				t, err := time.Parse("2006-01-02", dateFlag)
+				if err != nil {
+					return fmt.Errorf("invalid date %q (expected YYYY-MM-DD): %w", dateFlag, err)
+				}
+				targetDate = &t
+			}
+
+			if err := bsdaily.Run(targetDate); err != nil {
+				return err
+			}
+			slog.Info("completed successfully")
+			return nil
+		},
+	}
+
+	rootCmd.Flags().StringVarP(&dateFlag, "date", "d", "", "target date to extract (YYYY-MM-DD); defaults to snapshot date minus one day")
+
+	if err := rootCmd.Execute(); err != nil {
 		os.Exit(1)
 	}
-	slog.Info("completed successfully")
 }