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/utilities/_position.scss

@@ -1,50 +0,0 @@
-// Position
-.clearfix {
-  @include clearfix();
-}
-
-.float-left {
-  float: left !important;
-}
-
-.float-right {
-  float: right !important;
-}
-
-.relative {
-  position: relative !important;
-}
-
-.absolute {
-  position: absolute !important;
-}
-
-.fixed {
-  position: fixed !important;
-}
-
-.centered {
-  display: block;
-  float: none;
-  margin-left: auto;
-  margin-right: auto;
-}
-
-.flex-centered {
-  align-items: center;
-  display: flex;
-  justify-content: center;
-}
-
-// Spacing
-@include margin-variant(0, 0);
-
-@include margin-variant(1, $unit-1);
-
-@include margin-variant(2, $unit-2);
-
-@include padding-variant(0, 0);
-
-@include padding-variant(1, $unit-1);
-
-@include padding-variant(2, $unit-2);