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/flex-objects/composer.json

@@ -1,32 +0,0 @@
-{
-  "name": "getgrav/grav-plugin-flex-objects",
-  "type": "grav-plugin",
-  "description": "Flex Objects plugin for Grav CMS",
-  "keywords": ["flex-objects"],
-  "homepage": "https://github.com/trilbymedia/grav-plugin-flex-objects",
-  "license": "MIT",
-  "authors": [
-    {
-      "name": "Team Grav",
-      "email": "devs@getgrav.org",
-      "homepage": "http://getgrav.org",
-      "role": "Developer"
-    }
-  ],
-  "require": {
-    "php": "^7.3.6 || ^8.0",
-    "ext-json": "*",
-    "ext-mbstring": "*"
-  },
-  "autoload": {
-    "psr-4": {
-      "Grav\\Plugin\\FlexObjects\\": "classes/"
-    },
-    "classmap":  ["flex-objects.php"]
-  },
-  "config": {
-    "platform": {
-      "php": "7.3.6"
-    }
-  }
-}