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/_empty.scss

@@ -1,21 +0,0 @@
-// Empty states (or Blank slates)
-.empty {
-  background: $bg-color;
-  border-radius: $border-radius;
-  color: $gray-color-dark;
-  text-align: center;
-  padding: $unit-16 $unit-8;
-
-  .empty-icon {
-    margin-bottom: $layout-spacing-lg;
-  }
-
-  .empty-title,
-  .empty-subtitle {
-    margin: $layout-spacing auto;
-  }
-
-  .empty-action {
-    margin-top: $layout-spacing-lg;
-  }
-}