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>

test/test_version_comparison_consistency.py

@@ -9,9 +9,9 @@ can never disagree. (Historically the store used raw string equality, which
 reinstalled over cosmetic differences like "v1.2.0" vs "1.2.0" and even
 DOWNGRADED locally-ahead plugins; this file's tests killed that.)
 
-Two other version parsers legitimately remain and are pinned here so they
-don't drift: `compatibility.parse_semver` (the install-compatibility gate,
-range-spec oriented) and `skin_runtime._major` (skin API major gate).
+One other version parser legitimately remains and is pinned here so it
+doesn't drift: `compatibility.parse_semver` (the install-compatibility gate,
+range-spec oriented).
 """
 
 import json
@@ -21,7 +21,6 @@ import pytest
 from packaging.version import parse as pkg_parse
 
 from src.plugin_system.compatibility import is_update_available, parse_semver
-from src.skin_system.skin_runtime import _major
 from src.plugin_system.store_manager import PluginStoreManager
 from web_interface.blueprints.api_v3 import _is_plugin_update_available
 
@@ -142,25 +141,6 @@ class TestStoreManagerUsesSharedComparator:
         reinstall.assert_not_called()
 
 
-class TestSkinRuntimeMajor:
-    def test_plain_versions(self):
-        assert _major("1.0.0") == 1
-        assert _major("2.1") == 2
-
-    def test_int_input_tolerated(self):
-        assert _major(2) == 2
-
-    def test_garbage_returns_none(self):
-        assert _major("garbage") is None
-        assert _major(None) is None
-
-    def test_v_prefix_not_tolerated(self):
-        # Unlike parse_semver, _major does NOT strip a leading 'v' —
-        # a skin.json declaring "v1.0.0" fails the API gate. Characterized
-        # so a manifest-format loosening elsewhere doesn't silently diverge.
-        assert _major("v1.0.0") is None
-
-
 class TestParseSemverAgreesWithPackaging:
     """parse_semver and packaging must agree on ordering for plain X.Y.Z —
     the region where the two ecosystems overlap and must never diverge."""