Consolidate listing pages with unified grid cards and modal system (#2101)

* Prototype extension details modal

- Add detail popup modal for extension cards with full metadata and gallery
- Implement image gallery with thumbnail strip and main image selection
- Add modal styling and positioning in global.css
- Connect card click handlers to open modal with extension data

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

* Fix accessibility issues with modal focus restoration

- Add missing listing-cards-page class to agents.astro page root
- Pass focusable button element to openCardDetailsModal instead of article
- Fixes focus restoration for keyboard users when closing modal
- Applied fix across all listing pages (agents, instructions, hooks, plugins, skills, workflows)

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

* Address remaining PR review feedback

- Fix extension modal ARIA state by setting aria-current to "true" and removing it when inactive
- Use focusable .resource-preview as modal trigger for extension thumbnail/click/keyboard paths
- Extract shared multi-select helpers into pages/select-utils.ts and reuse across instructions/hooks/plugins/workflows
- Remove unused card-model.ts to avoid dead/overlapping type definitions

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

---------

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

website/src/pages/agents.astro

@@ -1,18 +1,18 @@
 ---
 import StarlightPage from '@astrojs/starlight/components/StarlightPage.astro';
 import agentsData from '../../public/data/agents.json';
-import Modal from '../components/Modal.astro';
 import ContributeCTA from '../components/ContributeCTA.astro';
 import EmbeddedPageData from '../components/EmbeddedPageData.astro';
 import PageHeader from '../components/PageHeader.astro';
 import BackToTop from '../components/BackToTop.astro';
+import Modal from '../components/Modal.astro';
 import { renderAgentsHtml, sortAgents } from '../scripts/pages/agents-render';
 
 const initialItems = sortAgents(agentsData.items, 'title');
 ---
 
 <StarlightPage frontmatter={{ title: 'Custom Agents', description: 'Specialized agents that enhance GitHub Copilot for specific technologies, workflows, and domains', template: 'splash', prev: false, next: false, editUrl: false }}>
-  <div id="main-content">
+  <div id="main-content" class="agents-page listing-cards-page">
     <PageHeader title="Custom Agents" description="Specialized agents that enhance GitHub Copilot for specific technologies, workflows, and domains" icon="robot" />
 
     <div class="page-content">
@@ -42,8 +42,8 @@ const initialItems = sortAgents(agentsData.items, 'title');
     </div>
   </div>
 
-  <Modal />
   <BackToTop />
+  <Modal />
   <EmbeddedPageData filename="agents.json" data={agentsData} />
 
   <script>