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/email/vendor/psr/event-dispatcher/composer.json

@@ -1,26 +0,0 @@
-{
-    "name": "psr/event-dispatcher",
-    "description": "Standard interfaces for event handling.",
-    "type": "library",
-    "keywords": ["psr", "psr-14", "events"],
-    "license": "MIT",
-    "authors": [
-        {
-            "name": "PHP-FIG",
-            "homepage": "http://www.php-fig.org/"
-        }
-    ],
-    "require": {
-        "php": ">=7.2.0"
-    },
-    "autoload": {
-        "psr-4": {
-            "Psr\\EventDispatcher\\": "src/"
-        }
-    },
-    "extra": {
-        "branch-alias": {
-            "dev-master": "1.0.x-dev"
-        }
-    }
-}