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/package.json

@@ -1,45 +0,0 @@
-{
-  "name": "spectre.css",
-  "version": "0.5.1",
-  "homepage": "http://picturepan2.github.io/spectre",
-  "author": "Yan Zhu <picturepan2@hotmail.com>",
-  "description": "Spectre.css: a lightweight, responsive and modern CSS framework",
-  "main": "docs/dist/spectre.css",
-  "repository": {
-    "type": "git",
-    "url": "https://github.com/picturepan2/spectre.git"
-  },
-  "license": "MIT",
-  "keywords": [
-    "css",
-    "framework",
-    "flexbox",
-    "responsive",
-    "mobile-friendly",
-    "front-end",
-    "sass",
-    "modern"
-  ],
-  "bugs": {
-    "url": "https://github.com/picturepan2/spectre/issues"
-  },
-  "devDependencies": {
-    "gulp": "^3.9.1",
-    "gulp-autoprefixer": "^7.0.1",
-    "gulp-clean-css": "^3.9.4",
-    "gulp-csscomb": "^3.0.8",
-    "gulp-rename": "^1.2.2",
-    "gulp-sass": "^4.0.1"
-  },
-  "browserslist": [
-    "last 4 Chrome versions",
-    "Edge >= 12",
-    "Firefox ESR",
-    "last 4 Safari versions",
-    "last 4 Opera versions",
-    "Explorer >= 10"
-  ],
-  "dependencies": {
-    "gulp-sourcemaps": "^2.6.4"
-  }
-}