add image-viewer card + clide image show verb

Drives an image inline into the Claude conversation log over the same
bus-publish path as ui.toast/ui.open, keeping the dispatcher handler
Flutter-free. The card is display-only per D-78; the verb registers a
CommandSchema so it surfaces in clide capabilities for T-248 discovery.

Closes T-249.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/claude/src/activity_cluster.dart

@@ -88,9 +88,11 @@ List<RenderGroup> groupConversation(List<ConversationItem> items, FoldLevel leve
 bool _isFoldable(ConversationItem item, FoldLevel level, Map<String, String> toolName) {
   if (level == FoldLevel.none) return false;
   switch (item) {
-    // User prose and Claude prose are always first-class.
+    // User prose, Claude prose, and driven-in image cards are always
+    // first-class — an image is the point of the turn, never folded away.
     case UserMessage():
     case AssistantTextMessage():
+    case ImageMessage():
       return false;
     // Thinking folds at L2+, first-class at L1.
     case AssistantThinkingMessage():