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/spectre.scss

@@ -1,51 +0,0 @@
-// Variables and mixins
-@import "variables";
-@import "mixins";
-
-/*! Spectre.css v#{$version} | MIT License | github.com/picturepan2/spectre */
-// Reset and dependencies
-@import "normalize";
-@import "base";
-
-// Elements
-@import "typography";
-@import "asian";
-@import "tables";
-@import "buttons";
-@import "forms";
-@import "labels";
-@import "codes";
-@import "media";
-
-// Layout
-@import "layout";
-@import "navbar";
-
-// Components
-@import "accordions";
-@import "avatars";
-@import "badges";
-@import "breadcrumbs";
-@import "bars";
-@import "cards";
-@import "chips";
-@import "dropdowns";
-@import "empty";
-@import "menus";
-@import "modals";
-@import "navs";
-@import "pagination";
-@import "panels";
-@import "popovers";
-@import "steps";
-@import "tabs";
-@import "tiles";
-@import "toasts";
-@import "tooltips";
-
-// Custom
-@import "problems";
-
-// Utility classes
-@import "animations";
-@import "utilities";