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/content.config.ts

@@ -1,20 +1,20 @@
 import { defineCollection, z } from "astro:content";
-import { glob } from "astro/loaders";
+import { docsLoader } from "@astrojs/starlight/loaders";
+import { docsSchema } from "@astrojs/starlight/schema";
 
-const learningHub = defineCollection({
-  loader: glob({ pattern: "**/*.md", base: "./src/content/learning-hub" }),
-  schema: z.object({
-    title: z.string(),
-    description: z.string(),
-    authors: z.array(z.string()).optional(),
-    lastUpdated: z.string().optional(),
-    estimatedReadingTime: z.string().optional(),
-    tags: z.array(z.string()).optional(),
-    relatedArticles: z.array(z.string()).optional(),
-    prerequisites: z.array(z.string()).optional(),
+const docs = defineCollection({
+  loader: docsLoader(),
+  schema: docsSchema({
+    extend: z.object({
+      authors: z.array(z.string()).optional(),
+      estimatedReadingTime: z.string().optional(),
+      tags: z.array(z.string()).optional(),
+      relatedArticles: z.array(z.string()).optional(),
+      prerequisites: z.array(z.string()).optional(),
+    }),
   }),
 });
 
 export const collections = {
-  "learning-hub": learningHub,
+  docs,
 };