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/utilities/_loading.scss

@@ -1,34 +0,0 @@
-// Loading
-.loading {
-  color: transparent !important;
-  min-height: $unit-4;
-  pointer-events: none;
-  position: relative;
-  &::after {
-    animation: loading 500ms infinite linear;
-    border: $border-width-lg solid $primary-color;
-    border-radius: 50%;
-    border-right-color: transparent;
-    border-top-color: transparent;
-    content: "";
-    display: block;
-    height: $unit-4;
-    left: 50%;
-    margin-left: -$unit-2;
-    margin-top: -$unit-2;
-    position: absolute;
-    top: 50%;
-    width: $unit-4;
-    z-index: $zindex-0;
-  }
-
-  &.loading-lg {
-    min-height: $unit-10;
-    &::after {
-      height: $unit-8;
-      margin-left: -$unit-4;
-      margin-top: -$unit-4;
-      width: $unit-8;
-    }
-  }
-}