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/_base.scss

@@ -1,44 +0,0 @@
-// Base
-*,
-*::before,
-*::after {
-  box-sizing: inherit;
-}
-
-html {
-  box-sizing: border-box;
-  font-size: $html-font-size;
-  line-height: $html-line-height;
-  -webkit-tap-highlight-color: transparent;
-}
-
-body {
-  background: $body-bg;
-  color: $body-font-color;
-  font-family: $body-font-family;
-  font-size: $font-size;
-  overflow-x: hidden;
-  text-rendering: optimizeLegibility;
-}
-
-a {
-  color: $link-color;
-  outline: none;
-  text-decoration: none;
-
-  &:focus {
-    @include control-shadow();
-  }
-
-  &:focus,
-  &:hover,
-  &:active,
-  &.active {
-    color: $link-color-dark;
-    text-decoration: underline;
-  }
-
-  &:visited {
-    color: $link-color-light;
-  }
-}