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>

docs/PLUGIN_API_REFERENCE.md

@@ -430,9 +430,7 @@ self.display_manager.image.paste(icon, (5, 5), icon)
 self.display_manager.update_display()
 ```
 
-This is the same pattern the bundled scoreboard base classes
-(`src/base_classes/baseball.py`, `basketball.py`, `football.py`,
-`hockey.py`) use, so it's the canonical way to render arbitrary images.
+This is the canonical way to render arbitrary images.
 
 ### Weather Icons