Deploy to a single target

There is only one server. The site is served from /var/www/mcg on the VPS
at mcg.luxick.de today, and the main domain will be pointed at the same
instance at cutover; local `hugo server` covers development.

Drops the development/production split from the workflow, its
DEPLOY_PATH_DEV / DEPLOY_PATH_PROD secrets and the workflow_dispatch
environment input, leaving a single DEPLOY_PATH. baseURL now comes from
hugo.toml rather than being injected per environment, so it is the one
place to change at cutover.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

hugo.toml

@@ -1,3 +1,5 @@
+# The live domain. Change this one line when the site moves to
+# https://motorradclub-giebelwald.de/ - it feeds canonical URLs, RSS and sitemap.
 baseURL = 'https://mcg.luxick.de/'
 title = 'Motorradclub Giebelwald e.V.'
 locale = 'de-DE'