fix: add @astrojs/markdown-remark dep and fix build-website CI paths (#2457)

* fix: add @astrojs/markdown-remark dep and fix build-website CI paths

Astro 7.1.x introduced 'Satteri' as the default Markdown processor and
no longer bundles @astrojs/markdown-remark by default. The website config
uses markdown.remarkPlugins which requires this package to be installed
explicitly.

Also update build-website.yml paths to use explicit ** glob patterns
and include root package.json/package-lock.json changes as triggers,
so dependabot package bumps always run the website build check.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: ed2219f7-f30c-4e24-95af-9c216166f285

* refactor: use explicit unified() processor in astro.config

Astro 7.1+ changed the markdown.remarkPlugins shorthand to require
@astrojs/markdown-remark. Update to the explicit unified() processor API
which is the supported way to use remark/rehype plugins in Astro 7.1+.

Sätteri (the new default processor) is incompatible with remark plugins,
so we stay on unified() to keep remark-github-admonitions-to-directives
working for Learning Hub GitHub admonition syntax.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: ed2219f7-f30c-4e24-95af-9c216166f285

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Copilot-Session: ed2219f7-f30c-4e24-95af-9c216166f285

.github/workflows/build-website.yml

@@ -4,14 +4,19 @@ on:
   pull_request:
     branches: [main]
     paths:
-      - website
-      - agents
-      - skills
-      - plugins
-      - instructions
-      - hooks
-      - workflows
-      - extensions
+      - "website/**"
+      - "agents/**"
+      - "skills/**"
+      - "plugins/**"
+      - "instructions/**"
+      - "hooks/**"
+      - "workflows/**"
+      - "extensions/**"
+      - "cookbook/**"
+      - "eng/**"
+      - ".all-contributorsrc"
+      - "package.json"
+      - "package-lock.json"
 
 permissions:
   contents: read