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/_accordions.scss

@@ -1,38 +0,0 @@
-// Accordions
-.accordion {
-  input:checked ~,
-  &[open] {
-    & .accordion-header {
-      .icon {
-        transform: rotate(90deg);
-      }
-    }
-
-    & .accordion-body {
-      max-height: 50rem;
-    }
-  }
-
-  .accordion-header {
-    display: block;
-    padding: $unit-1 $unit-2;
-
-    .icon {
-      transition: all .2s ease;
-    }
-  }
-
-  .accordion-body {
-    margin-bottom: $layout-spacing;
-    max-height: 0;
-    overflow: hidden;
-    transition: max-height .2s ease;
-  }
-}
-
-// Remove default details marker in Webkit
-summary.accordion-header {
-  &::-webkit-details-marker {
-    display: none;
-  }
-}