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/_timelines.scss

@@ -1,54 +0,0 @@
-// Timelines
-.timeline {
-  .timeline-item {
-    display: flex;
-    margin-bottom: $unit-6;
-    position: relative;
-    &::before {
-      background: $border-color;
-      content: "";
-      height: 100%;
-      left: 11px;
-      position: absolute;
-      top: $unit-6;
-      width: 2px;
-    }
-
-    .timeline-left {
-      flex: 0 0 auto;
-    }
-
-    .timeline-content {
-      flex: 1 1 auto;
-      padding: 2px 0 2px $layout-spacing-lg;
-    }
-
-    .timeline-icon {
-      border-radius: 50%;
-      color: $light-color;
-      display: block;
-      height: $unit-6;
-      text-align: center;
-      width: $unit-6;
-      &::before {
-        border: $border-width-lg solid $primary-color;
-        border-radius: 50%;
-        content: "";
-        display: block;
-        height: $unit-2;
-        left: $unit-2;
-        position: absolute;
-        top: $unit-2;
-        width: $unit-2;
-      }
-
-      &.icon-lg {
-        background: $primary-color;
-        line-height: $line-height;
-        &::before {
-          content: none;
-        }
-      }
-    }
-  }
-}