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/_display.scss

@@ -1,44 +0,0 @@
-// Display
-.d-block {
-  display: block;
-}
-.d-inline {
-  display: inline;
-}
-.d-inline-block {
-  display: inline-block;
-}
-.d-flex {
-  display: flex;
-}
-.d-inline-flex {
-  display: inline-flex;
-}
-.d-none,
-.d-hide {
-  display: none !important;
-}
-.d-visible {
-  visibility: visible;
-}
-.d-invisible {
-  visibility: hidden;
-}
-.text-hide {
-  background: transparent;
-  border: 0;
-  color: transparent;
-  font-size: 0;
-  line-height: 0;
-  text-shadow: none;
-}
-.text-assistive {
-  border: 0;
-  clip: rect(0,0,0,0);
-  height: 1px;
-  margin: -1px;
-  overflow: hidden;
-  padding: 0;
-  position: absolute;
-  width: 1px;
-}