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/_navs.scss

@@ -1,34 +0,0 @@
-// Navs
-.nav {
-  display: flex;
-  flex-direction: column;
-  list-style: none;
-  margin: $unit-1 0;
-
-  .nav-item {
-    a {
-      color: $gray-color-dark;
-      padding: $unit-1 $unit-2;
-      text-decoration: none;
-      &:focus,
-      &:hover {
-        color: $primary-color;
-      }
-    }
-    &.active {
-      & > a {
-        color: darken($gray-color-dark, 10%);
-        font-weight: bold;
-        &:focus,
-        &:hover {
-          color: $primary-color;
-        }
-      }
-    }
-  }
-
-  & .nav {
-    margin-bottom: $unit-2;
-    margin-left: $unit-4;
-  }
-}