Add blob manifest for pruning without decryption

- Update bucket structure to include unencrypted blob manifest files
- Add <snapshot_id>.manifest.json.zst containing list of referenced blobs
- This enables pruning operations without requiring decryption keys
- Add snapshot management commands: list, rm, latest (stubs)
- Add --prune flag to backup command for automatic cleanup
- Update DESIGN.md to document manifest format and updated prune flow

internal/cli/snapshot.go

@@ -0,0 +1,90 @@
+package cli
+
+import (
+	"github.com/spf13/cobra"
+)
+
+func SnapshotCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "snapshot",
+		Short: "Manage snapshots",
+		Long:  "Commands for listing, removing, and querying snapshots",
+	}
+
+	cmd.AddCommand(snapshotListCmd())
+	cmd.AddCommand(snapshotRmCmd())
+	cmd.AddCommand(snapshotLatestCmd())
+
+	return cmd
+}
+
+func snapshotListCmd() *cobra.Command {
+	var (
+		bucket string
+		prefix string
+		limit  int
+	)
+
+	cmd := &cobra.Command{
+		Use:   "list",
+		Short: "List snapshots",
+		Long:  "List all snapshots in the bucket, sorted by timestamp",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			panic("unimplemented")
+		},
+	}
+
+	cmd.Flags().StringVar(&bucket, "bucket", "", "S3 bucket name")
+	cmd.Flags().StringVar(&prefix, "prefix", "", "S3 prefix")
+	cmd.Flags().IntVar(&limit, "limit", 10, "Maximum number of snapshots to list")
+	cmd.MarkFlagRequired("bucket")
+
+	return cmd
+}
+
+func snapshotRmCmd() *cobra.Command {
+	var (
+		bucket   string
+		prefix   string
+		snapshot string
+	)
+
+	cmd := &cobra.Command{
+		Use:   "rm",
+		Short: "Remove a snapshot",
+		Long:  "Remove a snapshot and optionally its associated blobs",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			panic("unimplemented")
+		},
+	}
+
+	cmd.Flags().StringVar(&bucket, "bucket", "", "S3 bucket name")
+	cmd.Flags().StringVar(&prefix, "prefix", "", "S3 prefix")
+	cmd.Flags().StringVar(&snapshot, "snapshot", "", "Snapshot ID to remove")
+	cmd.MarkFlagRequired("bucket")
+	cmd.MarkFlagRequired("snapshot")
+
+	return cmd
+}
+
+func snapshotLatestCmd() *cobra.Command {
+	var (
+		bucket string
+		prefix string
+	)
+
+	cmd := &cobra.Command{
+		Use:   "latest",
+		Short: "Get the latest snapshot ID",
+		Long:  "Display the ID of the most recent snapshot",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			panic("unimplemented")
+		},
+	}
+
+	cmd.Flags().StringVar(&bucket, "bucket", "", "S3 bucket name")
+	cmd.Flags().StringVar(&prefix, "prefix", "", "S3 prefix")
+	cmd.MarkFlagRequired("bucket")
+
+	return cmd
+}