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/_popovers.scss

@@ -1,65 +0,0 @@
-// Popovers
-.popover {
-  display: inline-block;
-  position: relative;
-
-  .popover-container {
-    left: 50%;
-    opacity: 0;
-    padding: $layout-spacing;
-    position: absolute;
-    top: 0;
-    transform: translate(-50%, -50%) scale(0);
-    transition: transform .2s ease;
-    width: $control-width-sm;
-    z-index: $zindex-3;
-  }
-
-  *:focus + .popover-container,
-  &:hover .popover-container {
-    display: block;
-    opacity: 1;
-    transform: translate(-50%, -100%);
-  }
-
-  &.popover-right {
-    .popover-container {
-      left: 100%;
-      top: 50%;
-    }
-
-    *:focus + .popover-container,
-    &:hover .popover-container {
-      transform: translate(0, -50%);
-    }
-  }
-
-  &.popover-bottom {
-    .popover-container {
-      left: 50%;
-      top: 100%;
-    }
-
-    *:focus + .popover-container,
-    &:hover .popover-container {
-      transform: translate(-50%, 0);
-    }
-  }
-
-  &.popover-left {
-    .popover-container {
-      left: 0;
-      top: 50%;
-    }
-
-    *:focus + .popover-container,
-    &:hover .popover-container {
-      transform: translate(-100%, -50%);
-    }
-  }
-
-  .card {
-    @include shadow-variant(.2rem);
-    border: 0;
-  }
-}