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/_tiles.scss

@@ -1,38 +0,0 @@
-// Tiles
-.tile {
-  align-content: space-between;
-  align-items: flex-start;
-  display: flex;
-
-  .tile-icon,
-  .tile-action {
-    flex: 0 0 auto;
-  }
-  .tile-content {
-    flex: 1 1 auto;
-    &:not(:first-child) {
-      padding-left: $unit-2;
-    }
-    &:not(:last-child) {
-      padding-right: $unit-2;
-    }
-  }
-  .tile-title,
-  .tile-subtitle {
-    line-height: $line-height;
-  }
-
-  &.tile-centered {
-    align-items: center;
-
-    .tile-content {
-      overflow: hidden;
-    }
-
-    .tile-title,
-    .tile-subtitle {
-      @include text-ellipsis();
-      margin-bottom: 0;
-    }
-  }
-}