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/_cursors.scss

@@ -1,24 +0,0 @@
-// Cursors
-.c-hand {
-  cursor: pointer;
-}
-
-.c-move {
-  cursor: move;
-}
-
-.c-zoom-in {
-  cursor: zoom-in;
-}
-
-.c-zoom-out {
-  cursor: zoom-out;
-}
-
-.c-not-allowed {
-  cursor: not-allowed;
-}
-
-.c-auto {
-  cursor: auto;
-}