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/email.yaml

@@ -1,15 +0,0 @@
-enabled: true
-from:
-to:
-mailer:
-  engine: sendmail
-  smtp:
-    server: localhost
-    port: 25
-    encryption: none
-    user:
-    password:
-  sendmail:
-    bin: '/usr/sbin/sendmail -bs'
-content_type: text/html
-debug: false