Step 21: Lock/unlock toggle commands.

garden/lock.go: Lock() and Unlock() toggle the locked flag on
existing tracked entries. Errors on untracked paths. Persists
to manifest.

cmd/sgard/lock.go: sgard lock <path>..., sgard unlock <path>...

6 tests: lock/unlock existing entry, persistence, error on untracked,
checkpoint behavior changes after lock, status changes between
drifted and modified after unlock.

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

cmd/sgard/lock.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/kisom/sgard/garden"
+	"github.com/spf13/cobra"
+)
+
+var lockCmd = &cobra.Command{
+	Use:   "lock <path>...",
+	Short: "Mark tracked files as locked (repo-authoritative)",
+	Args:  cobra.MinimumNArgs(1),
+	RunE: func(cmd *cobra.Command, args []string) error {
+		g, err := garden.Open(repoFlag)
+		if err != nil {
+			return err
+		}
+
+		if err := g.Lock(args); err != nil {
+			return err
+		}
+
+		fmt.Printf("Locked %d path(s).\n", len(args))
+		return nil
+	},
+}
+
+var unlockCmd = &cobra.Command{
+	Use:   "unlock <path>...",
+	Short: "Remove locked flag from tracked files",
+	Args:  cobra.MinimumNArgs(1),
+	RunE: func(cmd *cobra.Command, args []string) error {
+		g, err := garden.Open(repoFlag)
+		if err != nil {
+			return err
+		}
+
+		if err := g.Unlock(args); err != nil {
+			return err
+		}
+
+		fmt.Printf("Unlocked %d path(s).\n", len(args))
+		return nil
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(lockCmd)
+	rootCmd.AddCommand(unlockCmd)
+}