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-idn/README.md

@@ -1,12 +0,0 @@
-Symfony Polyfill / Intl: Idn
-============================
-
-This component provides [`idn_to_ascii`](https://php.net/idn-to-ascii) and [`idn_to_utf8`](https://php.net/idn-to-utf8) functions to users who run php versions without the [Intl](https://php.net/intl) extension.
-
-More information can be found in the
-[main Polyfill README](https://github.com/symfony/polyfill/blob/main/README.md).
-
-License
-=======
-
-This library is released under the [MIT license](LICENSE).