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/symfony/polyfill-intl-normalizer/composer.json

@@ -1,36 +0,0 @@
-{
-    "name": "symfony/polyfill-intl-normalizer",
-    "type": "library",
-    "description": "Symfony polyfill for intl's Normalizer class and related functions",
-    "keywords": ["polyfill", "shim", "compatibility", "portable", "intl", "normalizer"],
-    "homepage": "https://symfony.com",
-    "license": "MIT",
-    "authors": [
-        {
-            "name": "Nicolas Grekas",
-            "email": "p@tchwork.com"
-        },
-        {
-            "name": "Symfony Community",
-            "homepage": "https://symfony.com/contributors"
-        }
-    ],
-    "require": {
-        "php": ">=7.2"
-    },
-    "autoload": {
-        "psr-4": { "Symfony\\Polyfill\\Intl\\Normalizer\\": "" },
-        "files": [ "bootstrap.php" ],
-        "classmap": [ "Resources/stubs" ]
-    },
-    "suggest": {
-        "ext-intl": "For best performance"
-    },
-    "minimum-stability": "dev",
-    "extra": {
-        "thanks": {
-            "name": "symfony/polyfill",
-            "url": "https://github.com/symfony/polyfill"
-        }
-    }
-}