Installs Go + Hugo extended and builds a Hugo site in preview
(--buildDrafts --buildFuture, development env) or production mode.
Extracted from the MCP repo's blog-preview, deploy-blog, and stage-blog
workflows where the ~12-line Go+Hugo install+build block was duplicated.
Placing it here (rather than as a local .github/actions/ composite in the
MCP repo) means stage-blog.yml can reference it at @main — fork-proof by
construction, no two-checkout dance needed when building fork-submitted
content.
