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/mixins/_position.scss

@@ -1,65 +0,0 @@
-// Margin utility mixin
-@mixin margin-variant($id: 1, $size: $unit-1) {
-  .m-#{$id} {
-    margin: $size !important;
-  }
-
-  .mb-#{$id} {
-    margin-bottom: $size !important;
-  }
-
-  .ml-#{$id} {
-    margin-left: $size !important;
-  }
-
-  .mr-#{$id} {
-    margin-right: $size !important;
-  }
-
-  .mt-#{$id} {
-    margin-top: $size !important;
-  }
-
-  .mx-#{$id} {
-    margin-left: $size !important;
-    margin-right: $size !important;
-  }
-
-  .my-#{$id} {
-    margin-bottom: $size !important;
-    margin-top: $size !important;
-  }
-}
-
-// Padding utility mixin
-@mixin padding-variant($id: 1, $size: $unit-1) {
-  .p-#{$id} {
-    padding: $size !important;
-  }
-
-  .pb-#{$id} {
-    padding-bottom: $size !important;
-  }
-
-  .pl-#{$id} {
-    padding-left: $size !important;
-  }
-
-  .pr-#{$id} {
-    padding-right: $size !important;
-  }
-
-  .pt-#{$id} {
-    padding-top: $size !important;
-  }
-
-  .px-#{$id} {
-    padding-left: $size !important;
-    padding-right: $size !important;
-  }
-  
-  .py-#{$id} {
-    padding-bottom: $size !important;
-    padding-top: $size !important;
-  }
-}