fix: disable oras tag fallback to tag schema when tagging a referrer

Signed-off-by: njucjc <njucjc@gmail.com>

cmd/oras/root/tag.go

@@ -22,6 +22,7 @@ import (
 	"github.com/spf13/cobra"
 	"oras.land/oras-go/v2"
 	"oras.land/oras-go/v2/errdef"
+	"oras.land/oras-go/v2/registry/remote"
 	"oras.land/oras/cmd/oras/internal/argument"
 	"oras.land/oras/cmd/oras/internal/command"
 	"oras.land/oras/cmd/oras/internal/display/status"
@@ -58,10 +59,6 @@ Example - Tag the manifest 'v1.0.1' in 'localhost:5000/hello' to 'v1.0.1', 'v1.0
 
 Example - Tag the manifest 'v1.0.1' to 'v1.0.2' in an OCI image layout folder 'layout-dir':
   oras tag --oci-layout layout-dir:v1.0.1 v1.0.2
-
-Example - Tag the referrers with digest sha256:9463e0d192846bc994279417b50114606712d516aab45f4d8b31cbc6e46aad71 to 'v1.0.2' using specific methods for the Referrers API:
-  oras tag --distribution-spec v1.1-referrers-api localhost:5000/hello@sha256:9463e0d192846bc994279417b50114606712d516aab45f4d8b31cbc6e46aad71 v1.0.2
-  oras tag --distribution-spec v1.1-referrers-tag localhost:5000/hello@sha256:9463e0d192846bc994279417b50114606712d516aab45f4d8b31cbc6e46aad71 v1.0.2
 `,
 		Args: func(cmd *cobra.Command, args []string) error {
 			if len(args) > 0 && (args[0] == "list" || args[0] == "ls") {
@@ -95,7 +92,6 @@ Example - Tag the referrers with digest sha256:9463e0d192846bc994279417b50114606
 		},
 	}
 
-	opts.EnableDistributionSpecFlag()
 	option.ApplyFlags(&opts, cmd.Flags())
 	cmd.Flags().IntVarP(&opts.concurrency, "concurrency", "", 5, "concurrency level")
 	return oerrors.Command(cmd, &opts.Target)
@@ -104,6 +100,12 @@ Example - Tag the referrers with digest sha256:9463e0d192846bc994279417b50114606
 func tagManifest(cmd *cobra.Command, opts *tagOptions) error {
 	ctx, logger := command.GetLogger(cmd, &opts.Common)
 	target, err := opts.NewTarget(opts.Common, logger)
+	if targetRepo, ok := target.(*remote.Repository); ok {
+		err := targetRepo.SetReferrersCapability(true)
+		if err != nil {
+			return err
+		}
+	}
 	if err != nil {
 		return err
 	}