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/_menus.scss

@@ -1,62 +0,0 @@
-// Menus
-.menu {
-  @include shadow-variant(.05rem);
-  background: $bg-color-light;
-  border-radius: $border-radius;
-  list-style: none;
-  margin: 0;
-  min-width: $control-width-xs;
-  padding: $unit-2;
-  transform: translateY($layout-spacing-sm);
-  z-index: $zindex-3;
-
-  &.menu-nav {
-    background: transparent;
-    box-shadow: none;
-  }
-
-  .menu-item {
-    margin-top: 0;
-    padding: 0 $unit-2;
-    text-decoration: none;
-    user-select: none;
-
-    & > a {
-      border-radius: $border-radius;
-      color: inherit;
-      display: block;
-      margin: 0 (-$unit-2);
-      padding: $unit-1 $unit-2;
-      text-decoration: none;
-      &:focus,
-      &:hover {
-        background: $secondary-color;
-        color: $primary-color;
-      }
-      &:active,
-      &.active {
-        background: $secondary-color;
-        color: $primary-color;
-      }
-    }
-
-    .form-checkbox,
-    .form-radio,
-    .form-switch {
-      margin: $unit-h 0;
-    }
-
-    & + .menu-item {
-      margin-top: $unit-1;
-    }
-  }
-
-  .menu-badge {
-    float: right;
-    padding: $unit-1 0;
-
-    .btn {
-      margin-top: -$unit-h;
-    }
-  }
-}