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/_chips.scss

@@ -1,30 +0,0 @@
-// Chips
-.chip {
-  align-items: center;
-  background: $bg-color-dark;
-  border-radius: 5rem;
-  color: $gray-color-dark;
-  display: inline-flex;
-  font-size: 90%;
-  height: $unit-6;
-  line-height: $unit-4;
-  margin: $unit-h;
-  max-width: 100%;
-  padding: $unit-1 $unit-2;
-  text-decoration: none;
-  vertical-align: middle;
-
-  &.active {
-    background: $primary-color;
-    color: $light-color;
-  }
-
-  .avatar {
-    margin-left: -$unit-2;
-    margin-right: $unit-1;
-  }
-
-  .btn-clear {
-    transform: scale(.75);
-  }
-}