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

website/package.json

@@ -19,6 +19,7 @@
   "author": "GitHub",
   "license": "MIT",
   "dependencies": {
+    "@astrojs/markdown-remark": "^7.2.0",
     "@astrojs/sitemap": "^3.7.2",
     "@astrojs/starlight": "^0.41.1",
     "astro": "^7.1.3",