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/container/composer.json

@@ -1,22 +0,0 @@
-{
-    "name": "psr/container",
-    "type": "library",
-    "description": "Common Container Interface (PHP FIG PSR-11)",
-    "keywords": ["psr", "psr-11", "container", "container-interop", "container-interface"],
-    "homepage": "https://github.com/php-fig/container",
-    "license": "MIT",
-    "authors": [
-        {
-            "name": "PHP-FIG",
-            "homepage": "https://www.php-fig.org/"
-        }
-    ],
-    "require": {
-        "php": ">=7.2.0"
-    },
-    "autoload": {
-        "psr-4": {
-            "Psr\\Container\\": "src/"
-        }
-    }
-}