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/_autocomplete.scss

@@ -1,47 +0,0 @@
-// Autocomplete
-.form-autocomplete {
-  position: relative;
-
-  .form-autocomplete-input {
-    align-content: flex-start;
-    display: flex;
-    flex-wrap: wrap;
-    height: auto;
-    min-height: $unit-8;
-    padding: $unit-h;
-
-    &.is-focused {
-      @include control-shadow();
-      border-color: $primary-color;
-    }
-
-    .form-input {
-      border-color: transparent;
-      box-shadow: none;
-      display: inline-block;
-      flex: 1 0 auto;
-      height: $unit-6;
-      line-height: $unit-4;
-      margin: $unit-h;
-      width: auto;
-    }
-  }
-
-  .menu {
-    left: 0;
-    position: absolute;
-    top: 100%;
-    width: 100%;
-  }
-
-  &.autocomplete-oneline {
-    .form-autocomplete-input {
-      flex-wrap: nowrap;
-      overflow-x: auto;
-    }
-
-    .chip {
-      flex: 1 0 auto;
-    }
-  }
-}