Add list command to display all tracked entries

Adds Garden.List() method that returns manifest entries, unit tests
for empty and populated repos, and a CLI command that formats output
by entry type (file with hash prefix, link with target, directory).

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

cmd/sgard/list.go

@@ -0,0 +1,41 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/kisom/sgard/garden"
+	"github.com/spf13/cobra"
+)
+
+var listCmd = &cobra.Command{
+	Use:   "list",
+	Short: "List all tracked files",
+	RunE: func(cmd *cobra.Command, args []string) error {
+		g, err := garden.Open(repoFlag)
+		if err != nil {
+			return err
+		}
+
+		entries := g.List()
+		for _, e := range entries {
+			switch e.Type {
+			case "file":
+				hash := e.Hash
+				if len(hash) > 8 {
+					hash = hash[:8]
+				}
+				fmt.Printf("%-6s %s\t%s\n", "file", e.Path, hash)
+			case "link":
+				fmt.Printf("%-6s %s\t-> %s\n", "link", e.Path, e.Target)
+			case "directory":
+				fmt.Printf("%-6s %s\n", "dir", e.Path)
+			}
+		}
+
+		return nil
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(listCmd)
+}