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/icons/_icons-core.scss

@@ -1,53 +0,0 @@
-// Icon variables
-$icon-border-width: $border-width-lg;
-$icon-prefix: "icon";
-
-// Icon base style
-.#{$icon-prefix} {
-  box-sizing: border-box;
-  display: inline-block;
-  font-size: inherit;
-  font-style: normal;
-  height: 1em;
-  position: relative;
-  text-indent: -9999px;
-  vertical-align: middle;
-  width: 1em;
-  &::before,
-  &::after {
-    display: block;
-    left: 50%;
-    position: absolute;
-    top: 50%;
-    transform: translate(-50%, -50%);
-  }
-
-  // Icon sizes
-  &.icon-2x {
-    font-size: 1.6rem;
-  }
-
-  &.icon-3x {
-    font-size: 2.4rem;
-  }
-
-  &.icon-4x {
-    font-size: 3.2rem;
-  }
-}
-
-// Component icon support
-.accordion,
-.btn,
-.toast,
-.menu {
-  .#{$icon-prefix} {
-    vertical-align: -10%;
-  }
-}
-
-.btn-lg {
-  .#{$icon-prefix} {
-    vertical-align: -15%;
-  }
-}