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/_asian.scss

@@ -1,38 +0,0 @@
-// Optimized for East Asian CJK
-:lang(zh),
-:lang(zh-Hans) {
-  font-family: $cjk-zh-hans-font-family;
-}
-
-:lang(zh-Hant) {
-  font-family: $cjk-zh-hant-font-family;
-}
-
-:lang(ja) {
-  font-family: $cjk-jp-font-family;
-}
-
-:lang(ko) {
-  font-family: $cjk-ko-font-family;
-}
-
-:lang(zh),
-:lang(ja),
-.cjk {
-  ins,
-  u {
-    border-bottom: $border-width solid;
-    text-decoration: none;
-  }
-
-  del + del,
-  del + s,
-  ins + ins,
-  ins + u,
-  s + del,
-  s + s,
-  u + ins,
-  u + u {
-    margin-left: .125em;
-  }
-}