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/problems/languages.yaml

@@ -1,14 +0,0 @@
-en:
-  PLUGIN_PROBLEMS:
-    BUILTIN_CSS: 'Use built in CSS'
-    BUILTIN_CSS_HELP: 'Include the CSS provided by the Problems plugin'
-
-ru:
-  PLUGIN_PROBLEMS:
-    BUILTIN_CSS: 'Использовать встроенный CSS'
-    BUILTIN_CSS_HELP: 'Использовать CSS, предоставленный плагином Problems'
-
-uk:
-  PLUGIN_PROBLEMS:
-    BUILTIN_CSS: 'Використовувати вбудований CSS'
-    BUILTIN_CSS_HELP: 'Використовувати CSS, наданий плагіном Problems'