Add verify command to check blob store integrity.

Verify iterates manifest file entries, confirms each blob exists in the
store, and re-hashes the content to detect corruption. Includes unit
tests for the ok, hash-mismatch, and blob-missing cases, plus a thin
CLI wrapper that exits non-zero on any failure.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cmd/sgard/verify.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/kisom/sgard/garden"
+	"github.com/spf13/cobra"
+)
+
+var verifyCmd = &cobra.Command{
+	Use:   "verify",
+	Short: "Check all blobs against manifest hashes",
+	RunE: func(cmd *cobra.Command, args []string) error {
+		g, err := garden.Open(repoFlag)
+		if err != nil {
+			return err
+		}
+
+		results, err := g.Verify()
+		if err != nil {
+			return err
+		}
+
+		allOK := true
+		for _, r := range results {
+			fmt.Printf("%-14s %s\n", r.Detail, r.Path)
+			if !r.OK {
+				allOK = false
+			}
+		}
+
+		if !allOK {
+			return errors.New("verification failed: one or more blobs are corrupt or missing")
+		}
+
+		return nil
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(verifyCmd)
+}