Remove prompts from website

Remove the prompts page, navigation link, homepage card, and all
references to prompts across the website source code. The repo no
longer contains prompt files, so the website should not reference them.

Files removed:
- website/src/pages/prompts.astro
- website/src/scripts/pages/prompts.ts

Files updated:
- BaseLayout.astro: remove nav link and update description
- index.astro: remove prompts card and update text
- llms.txt.ts: remove prompts import and section
- plugins.astro: update description text
- utils.ts: remove prompt type from install config, icons, labels
- pages/index.ts: remove prompts from counts
- modal.ts: update JSDoc comment

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/pages/plugins.astro

@@ -3,12 +3,12 @@ import BaseLayout from '../layouts/BaseLayout.astro';
 import Modal from '../components/Modal.astro';
 ---
 
-<BaseLayout title="Plugins" description="Curated plugins of prompts, agents, and skills for specific workflows" activeNav="plugins">
+<BaseLayout title="Plugins" description="Curated plugins of agents, instructions, and skills for specific workflows" activeNav="plugins">
   <main id="main-content">
     <div class="page-header">
       <div class="container">
         <h1>🔌 Plugins</h1>
-        <p>Curated plugins of prompts, agents, and skills for specific workflows</p>
+        <p>Curated plugins of agents, instructions, and skills for specific workflows</p>
       </div>
     </div>