refactor: remove the skin system and the unused src/base_classes package (#615)

* refactor: remove the skin system

Skins never rendered with the current scoreboard plugins: the only hook was
SportsCore._render_game in src/base_classes, which no plugin builds on, so
the UI and store already treated them as unsupported. The owner decided on
2026-09-23 to remove them outright.

Removed src/skin_system/ (runtime, base class, fixtures), skins/,
scripts/validate_skin.py and their tests; the store's "type": "skin"
installer, uninstaller and hide/refuse filters (the official registry lists
no skins); SchemaManager.inject_skin_selector; and GET /api/v3/skins.

Stored skin/skin_options config values are handled in the next commit.

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

* fix(config): drop retired skin/skin_options keys instead of validating them

A config.json written while the skin system existed can carry skin and
skin_options in any plugin section, and most plugin schemas set
additionalProperties: false. They are no longer core plugin properties;
RETIRED_PLUGIN_KEYS in schema_manager lists them and
drop_retired_plugin_keys removes them (unless the plugin's own schema
declares the name) in prepare_plugin_config, which loading, hot reload,
GET /plugins/config and both web saves already share, and in
validate_config_against_schema for callers that validate a raw section.
POST /plugins/config and /config/main also drop them from the stored
section they merge into, so they leave config.json on the next save.

Tests cover the load path (real PluginManager.load_plugin: no schema
warning, not degraded), raw and prepared validation,
validate_all_plugin_configs, and the JSON, form and /config/main saves.

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

* refactor: remove the unused src/base_classes package

No scoreboard plugin builds on src.base_classes: the nine monorepo
scoreboards ship their own sports.py and share code through src/common
(docs/SPORTS_UNIFICATION.md), and none of the third-party registry plugins
imports it. The one import anywhere, baseball-scoreboard's
rankings_manager.py, is a lazy import of ESPNDataSource in a class nothing
instantiates.

Removed the package and the eight test files that only tested it
(test_api_extractors, test_data_sources, test_sports_base_characterization,
test_sports_capabilities, test_sports_core_promotions,
test_sports_logo_cache_bounded, test_sports_modes_promotions,
test_sports_odds_fanout). test_common_is_hardware_free no longer lists
src.base_classes as a forbidden import, and comments in sports_helpers.py
and base_odds_manager.py stop pointing at it.

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

* docs: drop the skin system and src/base_classes from the docs

Deletes docs/SKIN_SYSTEM.md and docs/CREATING_SKINS.md and every link to
them (docs/README.md, README.md, PLUGIN_DEVELOPMENT_GUIDE.md, the /skins
section of REST_API_REFERENCE.md), the skin section of CLAUDE.md and the
term in PRODUCT.md. SPORTS_UNIFICATION.md now says src/base_classes was
removed and shared code lives in src/common, in the Layering section and
the view-model-contract rule. Other docs stop pointing at the removed
package. CHANGELOG records both removals under Unreleased.

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

* fix(store): hide and refuse registry entries that aren't plugins

The skin filters went with the skin system, but a custom registry can still
list "type": "skin" entries, and installing one as a plugin would unpack it
into the plugins directory. PluginStoreManager.is_plugin_entry() (a missing
type means plugin) now hides non-plugin entries from the store and
custom-registry listings, and install refuses them, in the route with a
clear 400 and in _install_plugin_impl for any other caller.

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

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/common/sports_helpers.py

@@ -2,9 +2,8 @@
 
 Ten helpers are byte-identical (executable AST, docstrings stripped) in the
 scoreboard plugins' ``sports.py`` and have no equivalent elsewhere in
-``src/common``. The bodies below were copied from those plugin copies -- not
-from ``src/base_classes`` -- at ledmatrix-plugins ``f09bff2`` (origin/main,
-2026-09-14):
+``src/common``. The bodies below were copied from those plugin copies at
+ledmatrix-plugins ``f09bff2`` (origin/main, 2026-09-14):
 
 - In all nine (afl, baseball, basketball, football, hockey, lacrosse, nrl,
   soccer, ufc): ``_clamp_window``, ``_clamp_seconds``, ``_logo_needs_refresh``
@@ -34,9 +33,9 @@ where both can. Plugins import this and floor ``ledmatrix_min_version`` on the
 first core release that ships it (see ``CHANGELOG.md``).
 
 ``_favorite_key`` is the one method not taken from the plugins: it is the
-override point from ``src/base_classes/sports/core.py``, carried here so later
-phases (shared celebrations and game selection) have a hardware-free home for
-the seam. No plugin defines it today and nothing in this module calls it.
+override point from the since-removed ``src/base_classes`` sports core,
+carried here so later phases (shared celebrations and game selection) have a
+hardware-free home for the seam. No plugin defines it today and nothing in this module calls it.
 
 WHAT A HOST MUST PROVIDE
 ------------------------
@@ -80,8 +79,8 @@ runs. By convention list it after ``SportsCoreSharedMixin``::
 
 HARDWARE-FREE
 -------------
-Nothing here may import ``src.display_manager``, ``src.base_classes``,
-``src.plugin_system`` or anything else that reaches ``rgbmatrix`` at module
+Nothing here may import ``src.display_manager``, ``src.plugin_system`` or
+anything else that reaches ``rgbmatrix`` at module
 level; ``test/test_common_is_hardware_free.py`` enforces that for all of
 ``src/common``. ``logo_needs_refresh`` imports ``src.logo_downloader`` lazily,
 exactly as the plugin copy does.
@@ -224,8 +223,8 @@ class SportsHelpersMixin:
         containing that string. The default returns ``None`` for a missing
         abbreviation, which never matches.
 
-        Carried from ``src/base_classes/sports/core.py`` for later phases;
-        nothing in this module calls it yet.
+        Carried from the since-removed ``src/base_classes`` sports core for
+        later phases; nothing in this module calls it yet.
         """
         return game.get(f"{side}_abbr")