feat: add internal/ui prompts + init subcommand

- internal/ui/prompts.go: MultiSelect with huh TUI + WithNonInteractive for tests
- internal/cli/init.go: NewInit cobra command with --from/--no-prompt flags
- internal/cli/init_test.go: TestInitImportWritesCanonical
- internal/cli/root.go: add resolveRoot helper (shared by profile/rollback/show/init)
- go.mod: add github.com/charmbracelet/huh v1.0.0 as direct dep

internal/cli/diff.go

@@ -9,15 +9,23 @@ import (
 // NewDiff returns the `diff` subcommand. It is a shorthand for
 // `apply --dry-run` and shows pending changes without writing.
 func NewDiff(reg *adapter.Registry) *cobra.Command {
-	return &cobra.Command{
+	var root string
+	cmd := &cobra.Command{
 		Use:   "diff [harness...]",
 		Short: "Show pending changes (alias for `apply --dry-run`)",
 		RunE: func(cmd *cobra.Command, args []string) error {
 			apply := NewApply(reg)
 			apply.SetOut(cmd.OutOrStdout())
 			apply.SetErr(cmd.ErrOrStderr())
-			apply.SetArgs(append([]string{"--dry-run"}, args...))
+			applyArgs := []string{"--dry-run"}
+			if root != "" {
+				applyArgs = append(applyArgs, "--root", root)
+			}
+			applyArgs = append(applyArgs, args...)
+			apply.SetArgs(applyArgs)
 			return apply.Execute()
 		},
 	}
+	cmd.Flags().StringVar(&root, "root", "", "canonical root (default ~/.config/harness-sync)")
+	return cmd
 }