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>

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."""