docs(plugin-config): match the config tab, icon and web-action docs to the code

- PLUGIN_CONFIG_QUICK_START / PLUGIN_CONFIGURATION_TABS /
  PLUGIN_CONFIGURATION_GUIDE: there is no "Reset to Defaults" button (the
  tab has Refresh, Update, Uninstall, Save Configuration); plugin config
  hot-reloads (ConfigService + on_config_change), so no restart; the
  schema is found by the fixed name config_schema.json, not a manifest
  config_schema field; the tab row is "Plugin Manager", not "Plugins";
  forms are server-rendered from /v3/partials/plugin-config/<id>; the
  duration hook is get_display_duration()/display_duration; a class_name
  mismatch raises PluginError; the store requires id, name, class_name and
  display_modes (not version); plugin_system.debug/log_level do not exist
  (use run.py -d / LEDMATRIX_DEBUG). Drop "future" features that shipped.
- PLUGIN_CONFIG_CORE_PROPERTIES: list all of CORE_PLUGIN_PROPERTIES,
  including skin, skin_options and the vegas_* tuning keys.
- PLUGIN_CUSTOM_ICONS: icon is only a Font Awesome class (fallback
  fa-puzzle-piece); emoji/URL icons and getPluginIcon() never existed in
  v3. Note that /api/v3/plugins/installed currently omits icon.
- PLUGIN_WEB_UI_ACTIONS (+ example JSON): success_message, error_message
  and step1_message are never read.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/PLUGIN_CONFIG_CORE_PROPERTIES.md

@@ -6,7 +6,8 @@ The LEDMatrix plugin system automatically manages certain core properties that a
 
 ## Core Properties
 
-The following properties are automatically managed by the system:
+The following properties are automatically managed by the system (the list
+is `CORE_PLUGIN_PROPERTIES` in `src/plugin_system/schema_manager.py`):
 
 1. **`enabled`** (boolean)
    - Default: `true`
@@ -24,6 +25,24 @@ The following properties are automatically managed by the system:
    - Description: Enable live priority takeover when plugin has live content
    - Used by DisplayController for priority scheduling
 
+4. **`skin`** (string, object or null; no default)
+   - Description: Visual skin id, or a per-mode mapping like `{"live": "my-skin"}`
+   - Not an enum, so a stored value keeps validating after the skin is
+     uninstalled. Skins do not render with the current scoreboard plugins yet
+     (see [SKIN_SYSTEM.md](SKIN_SYSTEM.md)); the key is kept so stored values
+     keep loading and saving
+
+5. **`skin_options`** (object; no default)
+   - Description: Options passed through to the selected skin
+
+6. **`vegas_width_pct`**, **`vegas_overflow`**, **`vegas_max_width_screens`**
+   (untyped; no default)
+   - Description: Vegas mode tuning for this plugin — card width as a
+     percentage of the panel, `"rotate"` or `"truncate"` on overflow, and the
+     widest the card may be in screens
+   - Read by `src/vegas_mode/plugin_adapter.py` and `BasePlugin`, which
+     validate the values themselves and ignore a bad one with a log line
+
 ## How Core Properties Work
 
 ### Schema Validation