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/_mixins.scss

@@ -1,11 +0,0 @@
-// Mixins
-@import "mixins/avatar";
-@import "mixins/button";
-@import "mixins/clearfix";
-@import "mixins/color";
-@import "mixins/label";
-@import "mixins/position";
-@import "mixins/shadow";
-@import "mixins/text";
-@import "mixins/toast";
-@import "mixins/transition";