Migrate website to Starlight with full-text resource search (#883)

* Add search functionality to Learning Hub index page

Add a client-side search bar that filters articles by title, description,
and tags. Sections with no matching results are hidden automatically.
Uses the existing .search-bar CSS pattern from the cookbook page.

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

* chore: remove deprecated layouts, theme script, and learning-hub config

Phase 5 cleanup of Starlight migration:
- Delete BaseLayout.astro (replaced by StarlightPage)
- Delete ArticleLayout.astro (replaced by Starlight docs rendering)
- Delete theme.ts (Starlight has built-in theme toggle)
- Delete src/config/learning-hub.ts (sidebar order now in astro.config.mjs)
- Replace learning-hub glob collection with Starlight docs collection in content.config.ts
- Keep search.ts (still used by homepage and all resource page scripts)

Build verified: 23 pages, no errors.

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

* Migrate website to Starlight with full-text resource search

- Replace bespoke Astro layouts with Starlight integration
  - Homepage and resource pages use StarlightPage wrapper
  - Learning Hub articles rendered via Starlight docs collection
  - Starlight provides search, theme toggle, sidebar, ToC, a11y

- Add custom Pagefind integration for resource search
  - All 614 agents/skills/instructions/hooks/workflows/plugins
    indexed as custom records with deep-link URLs
  - Type filter pills (horizontal pill toggles) above results
  - Search results link directly to resource modals via #file= hash

- Move global.css to src/styles/ for Vite processing
  - Scope CSS reset to #main-content to avoid Starlight conflicts
  - Full-width page gradient via body:has(#main-content)
  - Light/dark theme support with Starlight gray scale inversion

- Delete old layouts (BaseLayout, ArticleLayout), theme.ts, config

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

* Address PR review feedback

- Fix pagefind-resources.ts header comment (pagefind:true not false)
- Remove unused base variable in cookbook/index.astro
- Replace hardcoded /awesome-copilot/ paths with relative links in index.md
- Delete stale public/styles/global.css (source of truth is src/styles/)
- Replace fragile getBasePath() with Astro config base in pagefind integration
- Document pagefind:true reasoning in astro.config.mjs
- Use proper visually-hidden pattern + :focus-visible ring for filter pills
- Remove dead header/nav/theme CSS from global.css (~160 lines)

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

---------

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

website/src/pages/learning-hub/cookbook/index.astro

@@ -1,51 +1,35 @@
 ---
-import BaseLayout from '../../../layouts/BaseLayout.astro';
+import StarlightPage from '@astrojs/starlight/components/StarlightPage.astro';
 import Modal from '../../../components/Modal.astro';
-
-const base = import.meta.env.BASE_URL;
 ---
 
-<BaseLayout title="Cookbook" description="Code samples, recipes, and examples for building with GitHub Copilot" activeNav="learning-hub">
-  <main id="main-content">
-    <div class="page-header">
-      <div class="container">
-        <nav class="breadcrumb" aria-label="Breadcrumb">
-          <a href={`${base}learning-hub/`}>← Learning Hub</a>
-        </nav>
-        <h1>🍳 Cookbook</h1>
-        <p>Code samples, recipes, and examples for building with GitHub Copilot tools</p>
-      </div>
+<StarlightPage frontmatter={{ title: 'Cookbook', description: 'Code samples, recipes, and examples for building with GitHub Copilot' }}>
+  <div class="cookbook-page">
+    <div class="search-bar">
+      <label for="search-input" class="sr-only">Search recipes</label>
+      <input type="text" id="search-input" placeholder="Search recipes..." autocomplete="off">
     </div>
 
-    <div class="page-content">
-      <div class="container">
-        <div class="search-bar">
-          <label for="search-input" class="sr-only">Search recipes</label>
-          <input type="text" id="search-input" placeholder="Search recipes..." autocomplete="off">
-        </div>
-        
-        <div class="filters-bar" id="filters-bar">
-          <div class="filter-group">
-            <label for="filter-language">Language:</label>
-            <select id="filter-language" aria-label="Filter by language">
-              <option value="">All Languages</option>
-            </select>
-          </div>
-          <div class="filter-group">
-            <label for="filter-tag">Tags:</label>
-            <select id="filter-tag" multiple aria-label="Filter by tags"></select>
-          </div>
-          <button id="clear-filters" class="btn btn-secondary btn-small">Clear Filters</button>
-        </div>
-        
-        <div class="results-count" id="results-count" aria-live="polite"></div>
-        
-        <div id="samples-list" role="list">
-          <div class="loading" aria-live="polite">Loading samples...</div>
-        </div>
+    <div class="filters-bar" id="filters-bar">
+      <div class="filter-group">
+        <label for="filter-language">Language:</label>
+        <select id="filter-language" aria-label="Filter by language">
+          <option value="">All Languages</option>
+        </select>
       </div>
+      <div class="filter-group">
+        <label for="filter-tag">Tags:</label>
+        <select id="filter-tag" multiple aria-label="Filter by tags"></select>
+      </div>
+      <button id="clear-filters" class="btn btn-secondary btn-small">Clear Filters</button>
     </div>
-  </main>
+
+    <div class="results-count" id="results-count" aria-live="polite"></div>
+
+    <div id="samples-list" role="list">
+      <div class="loading" aria-live="polite">Loading samples...</div>
+    </div>
+  </div>
 
   <Modal />
 
@@ -281,4 +265,4 @@ const base = import.meta.env.BASE_URL;
   <script>
     import '../../../scripts/pages/samples';
   </script>
-</BaseLayout>
+</StarlightPage>