feat(ninja): prompt for tags in plugin init/edit flow

Add promptTagsWithDefault helper (mirrors promptCategoriesWithDefault) and
wire it into the init flow so upsertPluginMod receives real user-entered tags
instead of nil. ListTags RPC stub left with TODO(tags) marker for Task 12.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/ninja/cmd/plugin.go

@@ -360,6 +360,10 @@ func newPluginInitCmd() *cobra.Command {
 					return err
 				}
 			}
+			tags, err := promptTagsWithDefault(ctx, cli, scanner, kind, existing.Plugin.Tags)
+			if err != nil {
+				return err
+			}
 
 			createReq := &v1.CreatePluginRequest{
 				ScopeSlug:   scopeAPISlug(scope),
@@ -385,7 +389,7 @@ func newPluginInitCmd() *cobra.Command {
 			if private {
 				modScope = ""
 			}
-			if err := upsertPluginMod(modScope, name, displayName, description, kind, cats, nil, private); err != nil {
+			if err := upsertPluginMod(modScope, name, displayName, description, kind, cats, tags, private); err != nil {
 				return err
 			}
 			fmt.Println("plugin.mod updated")
@@ -600,6 +604,57 @@ func promptCategoriesWithDefault(ctx context.Context, cli *orchclient.Client, sc
 	return out, nil
 }
 
+// promptTagsWithDefault prompts the user for free-form tags. Best-effort fetches
+// the top-20 most-used tags via ListTags(kind) to surface popular suggestions;
+// if that call fails (offline, etc.) it falls back to a plain prompt with a
+// one-line warning. Empty input keeps `current`. Validates with NormalizeTags;
+// on error, prints the issue and reprompts (one re-try, then bails).
+func promptTagsWithDefault(ctx context.Context, cli *orchclient.Client, scanner *bufio.Scanner, kind string, current []string) ([]string, error) {
+	// Best-effort popular-tags lookup.
+	var popular string
+	// TODO(tags): re-enable once core regenerates proto bindings to expose v1.ListTagsRequest (plan Task 12).
+	// if resp, err := cli.Reg.ListTags(ctx, connect.NewRequest(&v1.ListTagsRequest{Kind: kind, Limit: 20})); err == nil {
+	// 	parts := make([]string, 0, len(resp.Msg.Tags))
+	// 	for _, t := range resp.Msg.Tags {
+	// 		parts = append(parts, fmt.Sprintf("%s (%d)", t.Tag, t.Count))
+	// 	}
+	// 	popular = strings.Join(parts, ", ")
+	// }
+	_ = ctx
+	_ = cli
+
+	for attempt := 0; attempt < 2; attempt++ {
+		if len(current) > 0 {
+			fmt.Printf("Tags (current: %s)\n", strings.Join(current, ", "))
+		} else {
+			fmt.Println("Tags (current: none)")
+		}
+		if popular != "" {
+			fmt.Printf("Popular: %s\n", popular)
+		} else if attempt == 0 {
+			fmt.Println("(could not fetch popular tags — offline or unauthenticated)")
+		}
+		fmt.Print("Enter comma-separated tags (blank to keep current): ")
+		if !scanner.Scan() {
+			return nil, fmt.Errorf("cancelled")
+		}
+		raw := strings.TrimSpace(scanner.Text())
+		if raw == "" {
+			return current, nil
+		}
+		parts := strings.Split(raw, ",")
+		out, err := core.NormalizeTags(parts)
+		if err == nil {
+			return out, nil
+		}
+		fmt.Printf("  %s\n", err.Error())
+		if attempt == 0 {
+			fmt.Println("  Try again:")
+		}
+	}
+	return nil, fmt.Errorf("tag input failed validation after 2 attempts")
+}
+
 // pickedSet returns a set of the given slugs for O(1) membership checks.
 func pickedSet(slugs []string) map[string]struct{} {
 	s := make(map[string]struct{}, len(slugs))