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/_panels.scss

@@ -1,23 +0,0 @@
-// Panels
-.panel {
-  border: $border-width solid $border-color;
-  border-radius: $border-radius;
-  display: flex;
-  flex-direction: column;
-
-  .panel-header,
-  .panel-footer {
-    flex: 0 0 auto;
-    padding: $layout-spacing-lg;
-  }
-
-  .panel-nav {
-    flex: 0 0 auto;
-  }
-
-  .panel-body {
-    flex: 1 1 auto;
-    overflow-y: auto;
-    padding: 0 $layout-spacing-lg;
-  }
-}