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/_text.scss

@@ -1,6 +0,0 @@
-// Text Ellipsis
-@mixin text-ellipsis() {
-  overflow: hidden;
-  text-overflow: ellipsis;
-  white-space: nowrap;
-}