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/_icons.scss

@@ -1,5 +0,0 @@
-// CSS Icons
-@import "icons/icons-core";
-@import "icons/icons-navigation";
-@import "icons/icons-action";
-@import "icons/icons-object";