feat(studio): replace Add with Ask agent in catalog cards

Remove the Add button and drag-and-drop from catalog cards — blocks
and components need agent-guided customization, not blind insertion.

Each card now shows a single "Ask agent" button that copies a rich,
category-specific prompt to clipboard with context about what the
block does and how to customize it (captions: transcribe + style,
transitions: place at cut point, data: replace values, etc.).

packages/studio/src/components/sidebar/LeftSidebar.tsx

@@ -84,7 +84,6 @@ export const LeftSidebar = memo(
       onLint,
       linting,
       onToggleCollapse,
-      onAddBlock,
       onPreviewBlock,
       takeoverContent,
     },
@@ -246,8 +245,8 @@ export const LeftSidebar = memo(
               </div>
             )}
 
-            {STUDIO_BLOCKS_PANEL_ENABLED && tab === "blocks" && onAddBlock && (
-              <BlocksTab onAddBlock={onAddBlock} onPreviewBlock={onPreviewBlock} />
+            {STUDIO_BLOCKS_PANEL_ENABLED && tab === "blocks" && (
+              <BlocksTab onPreviewBlock={onPreviewBlock} />
             )}
 
             {/* Lint button pinned at the bottom */}