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/admin/themes/grav/css-compiled/simple-fonts.css

@@ -1,13 +0,0 @@
-body, h5, h6,
-.badge, .note, .grav-mdeditor-preview,
-input, select, textarea, button, .selectize-input,
-h1, h2, h3, h4,
-.fontfamily-sans .CodeMirror pre,
-#admin-menu li, .form-tabs > label, .label {
-  font-family: "Helvetica Neue", "Helvetica", "Tahoma", "Geneva", "Arial", sans-serif; }
-
-.CodeMirror pre,
-code, kbd, pre, samp, .mono {
-  font-family: "SFMono-Regular", Consolas, "Liberation Mono", Menlo, Courier, monospace; }
-
-/*# sourceMappingURL=simple-fonts.css.map */