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/gulpfile.js

@@ -1,39 +0,0 @@
-var gulp = require('gulp');
-var sass = require('gulp-sass');
-var cleancss = require('gulp-clean-css');
-var csscomb = require('gulp-csscomb');
-var rename = require('gulp-rename');
-var autoprefixer = require('gulp-autoprefixer');
-var sourcemaps = require('gulp-sourcemaps');
-
-// configure the paths
-var watch_dir = './scss/*.scss';
-var src_dir = './scss/*.scss';
-var dest_dir = './css';
-
-var paths = {
-    source: src_dir
-};
-
-gulp.task('watch', function() {
-    gulp.watch(watch_dir, ['build']);
-});
-
-gulp.task('build', function() {
-    gulp.src(paths.source)
-        .pipe(sourcemaps.init())
-        .pipe(sass({outputStyle: 'compact', precision: 10})
-            .on('error', sass.logError)
-        )
-        .pipe(sourcemaps.write())
-        .pipe(autoprefixer())
-        .pipe(gulp.dest(dest_dir))
-        .pipe(csscomb())
-        .pipe(cleancss())
-        .pipe(rename({
-            suffix: '.min'
-        }))
-        .pipe(gulp.dest(dest_dir));
-});
-
-gulp.task('default', ['build']);