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/scss/_animations.scss

@@ -1,20 +0,0 @@
-// Animations
-@keyframes loading {
-  0% {
-    transform: rotate(0deg);
-  }
-  100% {
-    transform: rotate(360deg);
-  }
-}
-
-@keyframes slide-down {
-  0% {
-    opacity: 0;
-    transform: translateY(-$unit-8);
-  }
-  100% {
-    opacity: 1;
-    transform: translateY(0);
-  }
-}