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/docs/learning-hub/agentic-workflows.md

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+../../learning-hub/agentic-workflows.md

website/src/content/docs/learning-hub/automating-with-hooks.md

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+../../learning-hub/automating-with-hooks.md

website/src/content/docs/learning-hub/before-after-customization-examples.md

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+../../learning-hub/before-after-customization-examples.md

website/src/content/docs/learning-hub/building-custom-agents.md

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+../../learning-hub/building-custom-agents.md

website/src/content/docs/learning-hub/copilot-configuration-basics.md

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+../../learning-hub/copilot-configuration-basics.md

website/src/content/docs/learning-hub/creating-effective-skills.md

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+../../learning-hub/creating-effective-skills.md

website/src/content/docs/learning-hub/defining-custom-instructions.md

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+../../learning-hub/defining-custom-instructions.md

website/src/content/docs/learning-hub/github-copilot-terminology-glossary.md

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+../../learning-hub/github-copilot-terminology-glossary.md

website/src/content/docs/learning-hub/index.md

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+---
+title: Learning Hub
+description: 'Curated articles, walkthroughs, and reference material to help you unlock everything you can do with GitHub Copilot'
+template: splash
+hero:
+  tagline: 'Curated articles, walkthroughs, and reference material to help you unlock everything you can do with GitHub Copilot'
+  actions:
+    - text: Start with Fundamentals
+      link: /learning-hub/what-are-agents-skills-instructions/
+      icon: right-arrow
+sidebar:
+  hidden: true
+tableOfContents: false
+---
+
+## Fundamentals
+
+Essential concepts to tailor GitHub Copilot beyond its default experience. Start with
+[What are Agents, Skills, and Instructions](/learning-hub/what-are-agents-skills-instructions/)
+and work through the full track to master every customization primitive.
+
+## Reference
+
+Quick-lookup resources to keep handy while you work. Browse the
+[GitHub Copilot Terminology Glossary](/learning-hub/github-copilot-terminology-glossary/)
+for definitions of common terms and concepts.
+
+## Hands-on
+
+Interactive samples and recipes to learn by doing. Jump into the
+[Cookbook](/learning-hub/cookbook/) for code samples, recipes,
+and examples you can use right away.

website/src/content/docs/learning-hub/installing-and-using-plugins.md

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+../../learning-hub/installing-and-using-plugins.md

website/src/content/docs/learning-hub/understanding-copilot-context.md

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+../../learning-hub/understanding-copilot-context.md

website/src/content/docs/learning-hub/understanding-mcp-servers.md

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+../../learning-hub/understanding-mcp-servers.md

website/src/content/docs/learning-hub/using-copilot-coding-agent.md

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+../../learning-hub/using-copilot-coding-agent.md

website/src/content/docs/learning-hub/what-are-agents-skills-instructions.md

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+../../learning-hub/what-are-agents-skills-instructions.md