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/_pagination.scss

@@ -1,61 +0,0 @@
-// Pagination
-.pagination {
-  display: flex;
-  list-style: none;
-  margin: $unit-1 0;
-  padding: $unit-1 0;
-
-  .page-item {
-    margin: $unit-1 $unit-o;
-
-    span {
-      display: inline-block;
-      padding: $unit-1 $unit-1;
-    }
-
-    a {
-      border-radius: $border-radius;
-      color: $gray-color-dark;
-      display: inline-block;
-      padding: $unit-1 $unit-2;
-      text-decoration: none;
-      &:focus,
-      &:hover {
-        color: $primary-color;
-      }
-    }
-
-    &.disabled {
-      a {
-        cursor: default;
-        opacity: .5;
-        pointer-events: none;
-      }
-    }
-
-    &.active {
-      a {
-        background: $primary-color;
-        color: $light-color;
-      }
-    }
-
-    &.page-prev,
-    &.page-next {
-      flex: 1 0 50%;
-    }
-
-    &.page-next {
-      text-align: right;
-    }
-
-    .page-item-title {
-      margin: 0;
-    }
-
-    .page-item-subtitle {
-      margin: 0;
-      opacity: .5;
-    }
-  }
-}