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/scripts/theme.ts

@@ -1,65 +0,0 @@
-/**
- * Theme management for the Awesome Copilot website
- * Supports light/dark mode with user preference storage
- */
-
-const THEME_KEY = 'theme';
-
-/**
- * Get the current theme preference
- */
-function getThemePreference(): 'light' | 'dark' {
-  const stored = localStorage.getItem(THEME_KEY);
-  if (stored === 'light' || stored === 'dark') {
-    return stored;
-  }
-  // Check system preference
-  if (window.matchMedia && window.matchMedia('(prefers-color-scheme: light)').matches) {
-    return 'light';
-  }
-  return 'dark';
-}
-
-/**
- * Apply theme to the document
- */
-function applyTheme(theme: 'light' | 'dark'): void {
-  document.documentElement.setAttribute('data-theme', theme);
-}
-
-const initialTheme = getThemePreference();
-applyTheme(initialTheme);
-
-/**
- * Toggle between light and dark theme
- */
-export function toggleTheme(): void {
-  const current = document.documentElement.getAttribute('data-theme') as 'light' | 'dark';
-  const newTheme = current === 'light' ? 'dark' : 'light';
-  applyTheme(newTheme);
-  localStorage.setItem(THEME_KEY, newTheme);
-}
-
-/**
- * Initialize theme toggle button
- */
-export function initThemeToggle(): void {
-  const toggleBtn = document.getElementById('theme-toggle');
-  if (toggleBtn) {
-    toggleBtn.addEventListener('click', toggleTheme);
-  }
-
-  // Listen for system theme changes
-  if (window.matchMedia) {
-    window.matchMedia('(prefers-color-scheme: light)').addEventListener('change', (e) => {
-      // Only auto-switch if user hasn't set a preference
-      const stored = localStorage.getItem(THEME_KEY);
-      if (!stored) {
-        applyTheme(e.matches ? 'light' : 'dark');
-      }
-    });
-  }
-}
-
-// Auto-initialize when DOM is ready
-document.addEventListener('DOMContentLoaded', initThemeToggle);