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/sitemap.yaml

@@ -1,25 +0,0 @@
-enabled: true
-route: '/sitemap'
-ignore_external: true
-ignore_protected: true
-ignore_redirect: true
-include_news_tags: false
-standalone_sitemap_news: false
-sitemap_news_path: '/sitemap-news.xml'
-news_max_age_days: 2
-news_enabled_paths:
-  - /blog
-ignores:
-whitelist:
-xsl_transform: true
-html_support: false
-urlset: 'http://www.sitemaps.org/schemas/sitemap/0.9'
-urlnewsset: 'http://www.google.com/schemas/sitemap-news/0.9'
-urlimageset: 'http://www.google.com/schemas/sitemap-image/1.1'
-short_date_format: true
-include_changefreq: true
-changefreq: daily
-include_priority: true
-priority: !!float 1
-additions: []
-multilang_enabled: true