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.lock

@@ -1,25 +0,0 @@
-{
-    "_readme": [
-        "This file locks the dependencies of your project to a known state",
-        "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
-        "This file is @generated automatically"
-    ],
-    "content-hash": "8cd40668f2f51e9d1c7bceed7be5d27d",
-    "packages": [],
-    "packages-dev": [],
-    "aliases": [],
-    "minimum-stability": "stable",
-    "stability-flags": [],
-    "prefer-stable": false,
-    "prefer-lowest": false,
-    "platform": {
-        "php": "^7.3.6 || ^8.0",
-        "ext-json": "*",
-        "ext-mbstring": "*"
-    },
-    "platform-dev": [],
-    "platform-overrides": {
-        "php": "7.3.6"
-    },
-    "plugin-api-version": "2.2.0"
-}