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/_labels.scss

@@ -1,34 +0,0 @@
-// Labels
-.label {
-  @include label-base();
-  @include label-variant(lighten($body-font-color, 5%), $bg-color-dark);
-  display: inline-block;
-
-  // Label rounded
-  &.label-rounded {
-    border-radius: 5rem;
-    padding-left: .4rem;
-    padding-right: .4rem; 
-  }
-
-  // Label colors
-  &.label-primary {
-    @include label-variant($light-color, $primary-color);
-  }
-
-  &.label-secondary {
-    @include label-variant($primary-color, $secondary-color);
-  }
-
-  &.label-success {
-    @include label-variant($light-color, $success-color);
-  }
-
-  &.label-warning {
-    @include label-variant($light-color, $warning-color);
-  }
-
-  &.label-error {
-    @include label-variant($light-color, $error-color);
-  }
-}