Move plugins to manifest, pin Docker version, add Makefile

- Add plugins.txt listing all plugins for reproducible installs
- Add Makefile with setup/start/stop/install-plugins targets
- Remove user/plugins/ from git tracking
- Pin Docker image to 1.7.49.5-ls244

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/sitemap/templates/sitemap-extensions/news.xml.twig

@@ -1,8 +0,0 @@
-<news:news>
-  <news:publication>
-    <news:name>{{ site.title }}</news:name>
-    <news:language>{{ entry.lang }}</news:language>
-  </news:publication>
-  <news:publication_date>{{ entry.longdate }}</news:publication_date>
-  <news:title>{{ entry.title }}</news:title>
-</news:news>