Phase 2: Refactor Prompts cluster to MCP SDK types

Replace ad hoc local types with MCP SDK schema types across the Prompts
group components and PromptsScreen, per the v2 UX interfaces plan.

- PromptListItem: accept `prompt: Prompt` instead of flat `name`/`description`;
  render `title ?? name` as primary label
- PromptControls: accept `Prompt[]` + `selectedName` instead of `PromptItem[]`;
  no-op on clicking already-selected prompt; search includes title field
- PromptArgumentsForm: accept `prompt: Prompt` instead of flat props;
  read `prompt.arguments` directly; display `title ?? name` as heading
- PromptMessagesDisplay: narrow type from `(PromptMessage | SamplingMessage)[]`
  to `PromptMessage[]` per spec
- PromptsScreen: accept `Prompt[]` + `selectedPromptName` + `GetPromptState`;
  derive selected prompt via `prompts.find()`; own `argumentValues` state
  locally; reset arguments on tool switch
- Delete local `PromptItem`, `SelectedPrompt`, `PromptArgument` types
- Update all stories with real SDK `Prompt` fixtures
- Update ConnectedView PromptsActive story

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

clients/web/src/components/groups/PromptMessagesDisplay/PromptMessagesDisplay.stories.tsx

@@ -52,17 +52,11 @@ export const WithImage: Story = {
     messages: [
       {
         role: "user",
-        content: [
-          {
-            type: "text",
-            text: "Here is a photo for you to analyze.",
-          },
-          {
-            type: "image",
-            data: "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==",
-            mimeType: "image/png",
-          },
-        ],
+        content: {
+          type: "image",
+          data: "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==",
+          mimeType: "image/png",
+        },
       },
     ],
   },