feat(website): add comprehensive accessibility improvements

Phase 1 - Screen Reader Critical:
- Add aria-label to main navigation
- Add accessible names to icon-only buttons (GitHub, theme toggle, close)
- Add aria-hidden to decorative SVGs and emoji icons
- Add role=dialog, aria-modal, aria-labelledby to modal
- Add skip link with visible focus state

Phase 2 - Keyboard Navigation:
- Implement focus trap in modal (Tab/Shift+Tab cycles)
- Return focus to trigger element on modal close
- Replace outline:none with visible focus rings
- Add keyboard navigation to install dropdown (arrows, escape)
- Add aria-expanded to dropdown toggles

Phase 3 - Dynamic Content:
- Add aria-live=polite to results counts and loading states
- Add role=listbox to search results
- Add role=list to resource lists
- Add role=menu/menuitem to dropdown menus

Phase 4 - Forms & Labels:
- Add .sr-only utility class for screen reader text
- Add visually hidden labels to all search inputs
- Add aria-label to filter dropdowns

Files modified:
- BaseLayout.astro, Modal.astro (ARIA attributes)
- modal.ts (focus trap, keyboard navigation)
- global.css (sr-only, skip-link, focus styles)
- All page files (labels, live regions, roles)

website/src/pages/instructions.astro

@@ -4,7 +4,7 @@ import Modal from '../components/Modal.astro';
 ---
 
 <BaseLayout title="Instructions" description="Coding standards and best practices for GitHub Copilot" activeNav="instructions">
-  <main>
+  <main id="main-content">
     <div class="page-header">
       <div class="container">
         <h1>📋 Instructions</h1>
@@ -15,20 +15,21 @@ import Modal from '../components/Modal.astro';
     <div class="page-content">
       <div class="container">
         <div class="search-bar">
+          <label for="search-input" class="sr-only">Search instructions</label>
           <input type="text" id="search-input" placeholder="Search instructions..." autocomplete="off">
         </div>
         
         <div class="filters-bar" id="filters-bar">
           <div class="filter-group">
             <label for="filter-extension">File Extension:</label>
-            <select id="filter-extension" multiple></select>
+            <select id="filter-extension" multiple aria-label="Filter by file extension"></select>
           </div>
           <button id="clear-filters" class="btn btn-secondary btn-small">Clear Filters</button>
         </div>
         
-        <div class="results-count" id="results-count"></div>
-        <div class="resource-list" id="resource-list">
-          <div class="loading">Loading instructions...</div>
+        <div class="results-count" id="results-count" aria-live="polite"></div>
+        <div class="resource-list" id="resource-list" role="list">
+          <div class="loading" aria-live="polite">Loading instructions...</div>
         </div>
       </div>
     </div>