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/_codes.scss

@@ -1,31 +0,0 @@
-// Codes
-code {
-  @include label-base();
-  @include label-variant($code-color, lighten($code-color, 42.5%));
-  font-size: 85%;
-}
-
-.code {
-  border-radius: $border-radius;
-  color: $body-font-color;
-  position: relative;
-
-  &::before {
-    color: $gray-color;
-    content: attr(data-lang);
-    font-size: $font-size-sm;
-    position: absolute;
-    right: $layout-spacing;
-    top: $unit-h;
-  }
-
-  code {
-    background: $bg-color;
-    color: inherit;
-    display: block;
-    line-height: 1.5;
-    overflow-x: auto;
-    padding: 1rem;
-    width: 100%;
-  }
-}