fix(HAI-039): eliminate duplicate description in prompts and modal heading

- Fix prompt heading to use only task ID when title is absent, preventing description duplication
- Strip leading markdown heading from TaskDetailModal since the modal has its own header
- Add store tests covering prompt generation for tasks with and without titles
- Update TaskDetailModal tests to verify heading stripping and single description rendering

packages/dashboard/app/components/TaskDetailModal.tsx

@@ -154,7 +154,7 @@ export function TaskDetailModal({
             {task.prompt ? (
               <div className="markdown-body">
                 <ReactMarkdown remarkPlugins={[remarkGfm]}>
-                  {task.prompt}
+                  {task.prompt.replace(/^#\s+[^\n]*\n+/, "")}
                 </ReactMarkdown>
               </div>
             ) : (