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/form/package.json

@@ -1,48 +0,0 @@
-{
-  "name": "grav-plugin-form",
-  "version": "1.0.0",
-  "description": "Grav Plugin Form",
-  "repository": "https://github.com/getgrav/grav-plugin-form",
-  "main": "app/main.js",
-  "scripts": {
-    "watch": "NODE_ENV=development webpack --watch --progress --colors --mode development --config webpack.conf.js",
-    "dev": "NODE_ENV=development webpack --progress --colors --config webpack.conf.js",
-    "prod": "NODE_ENV=production-wip webpack --mode development --config webpack.conf.js"
-  },
-  "author": "Team Grav",
-  "license": "MIT",
-  "dependencies": {
-    "dropzone": "getgrav/dropzone#master",
-    "exif-js": "^2.3.0",
-    "filepond": "^4.32.7",
-    "filepond-plugin-file-validate-size": "^2.2.8",
-    "filepond-plugin-file-validate-type": "^1.2.9",
-    "filepond-plugin-image-preview": "^4.6.12",
-    "filepond-plugin-image-resize": "^2.0.10",
-    "sortablejs": "^1.10.2"
-  },
-  "devDependencies": {
-    "@babel/core": "^7.9.0",
-    "@babel/preset-env": "^7.9.0",
-    "babel-eslint": "^10.1.0",
-    "babel-loader": "^8.1.0",
-    "babel-polyfill": "^6.26.0",
-    "css-loader": "^7.1.2",
-    "eslint": "^6.8.0",
-    "eslint-loader": "^3.0.4",
-    "exports-loader": "^0.7.0",
-    "gulp": "^5.0.0",
-    "gulp-autoprefixer": "^9.0.0",
-    "gulp-clean-css": "^4.3.0",
-    "gulp-csscomb": "^0.1.0",
-    "gulp-rename": "^2.0.0",
-    "gulp-webpack": "^0.0.1",
-    "immutable": "^4.0.0-rc.12",
-    "imports-loader": "^0.8.0",
-    "json-loader": "^0.5.7",
-    "style-loader": "^1.1.3",
-    "terser-webpack-plugin": "^5.3.10",
-    "webpack": "^5.98.0",
-    "webpack-cli": "^6.0.1"
-  }
-}