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/problems/scss/_meters.scss

@@ -1,57 +0,0 @@
-// Meters
-// Credit: https://css-tricks.com/html5-meter-element/
-.meter {
-  appearance: none;
-  background: $bg-color;
-  border: 0;
-  border-radius: $border-radius;
-  display: block;
-  width: 100%;
-  height: $unit-4;
-
-  &::-webkit-meter-inner-element {
-    display: block;
-  }
-
-  &::-webkit-meter-bar,
-  &::-webkit-meter-optimum-value,
-  &::-webkit-meter-suboptimum-value,
-  &::-webkit-meter-even-less-good-value {
-    border-radius: $border-radius;
-  }
-
-  &::-webkit-meter-bar {
-    background: $bg-color;
-  }
-
-  &::-webkit-meter-optimum-value {
-    background: $success-color;
-  }
-
-  &::-webkit-meter-suboptimum-value {
-    background: $warning-color;
-  }
-
-  &::-webkit-meter-even-less-good-value {
-    background: $error-color;
-  }
-
-  &::-moz-meter-bar,
-  &:-moz-meter-optimum,
-  &:-moz-meter-sub-optimum,
-  &:-moz-meter-sub-sub-optimum {
-    border-radius: $border-radius;
-  }
-
-  &:-moz-meter-optimum::-moz-meter-bar {
-    background: $success-color;
-  }
-
-  &:-moz-meter-sub-optimum::-moz-meter-bar {
-    background: $warning-color;
-  }
-
-  &:-moz-meter-sub-sub-optimum::-moz-meter-bar {
-    background: $error-color;
-  }
-}