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/sitemap/composer.json

@@ -1,30 +0,0 @@
-{
-  "name": "trilbymedia/sitemap",
-  "type": "grav-sitemap",
-  "description": "Provide automatically generated **XML sitemaps** with this very useful, but simple to configure, Grav plugin.",
-  "keywords": ["plugin"],
-  "homepage": "https://github.com/getgrav/grav-plugin-sitemap",
-  "license": "MIT",
-  "authors": [
-    {
-      "name": "Team Grav",
-      "email": "devs@getgrav.org",
-      "homepage": "https://getgrav.org",
-      "role": "Developer"
-    }
-  ],
-  "require": {
-    "php": ">=7.4"
-  },
-  "autoload": {
-    "psr-4": {
-      "Grav\\Plugin\\Sitemap\\": "classes/"
-    },
-    "classmap":  ["sitemap.php"]
-  },
-  "config": {
-    "platform": {
-      "php": "7.4.33"
-    }
-  }
-}