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/composer.json

@@ -1,45 +0,0 @@
-{
-    "name": "getgrav/grav-plugin-email",
-    "type": "grav-plugin",
-    "description": "Email plugin for Grav CMS",
-    "keywords": ["email", "plugin", "sender"],
-    "homepage": "https://github.com/getgrav/grav-plugin-email",
-    "license": "MIT",
-    "authors": [
-        {
-            "name": "Team Grav",
-            "email": "devs@getgrav.org",
-            "homepage": "https://getgrav.org",
-            "role": "Developer"
-        }
-    ],
-    "support": {
-        "issues": "https://github.com/getgrav/grav-plugin-email/issues",
-        "irc": "https://chat.getgrav.org",
-        "forum": "https://getgrav.org/forum",
-        "docs": "https://github.com/getgrav/grav-plugin-email/blob/master/README.md"
-    },
-    "require": {
-        "php": ">=7.3.6",
-        "symfony/mailer": "^5.4",
-        "symfony/messenger": "^5.4"
-    },
-    "replace": {
-        "symfony/polyfill-iconv": "*",
-        "symfony/polyfill-mbstring": "*",
-        "symfony/polyfill-php72": "*",
-        "psr/log": "*"
-    },
-    "autoload": {
-        "psr-4": {
-            "Grav\\Plugin\\Email\\": "classes/",
-            "Grav\\Plugin\\Console\\": "cli/"
-        },
-        "classmap":  ["email.php"]
-    },
-    "config": {
-        "platform": {
-            "php": "7.3.6"
-        }
-    }
-}