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/draft-preview/composer.json

@@ -1,29 +0,0 @@
-{
-  "name": "bitstarr/draft-preview",
-  "type": "grav-plugin",
-  "description": "Preview Pages that are not published yet",
-  "keywords": ["plugin"],
-  "homepage": "https://github.com/bitstarr/grav-plugin-draft-preview",
-  "license": "MIT",
-  "authors": [
-    {
-      "name": "Sebastian Laube",
-      "email": "hello@sebastianlaube.de",
-      "role": "Developer"
-    }
-  ],
-  "require": {
-    "php": ">=7.1.3"
-  },
-  "autoload": {
-    "psr-4": {
-      "Grav\\Plugin\\DraftPreview\\": "classes/"
-    },
-    "classmap":  ["draft-preview.php"]
-  },
-  "config": {
-    "platform": {
-      "php": "7.1.3"
-    }
-  }
-}