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>

CLAUDE.md

@@ -45,17 +45,6 @@
 - Plugin configs stored in `config/config.json`, NOT in plugin directories — safe across reinstalls
 - Third-party plugins can use their own repo URL with empty `plugin_path`
 
-## Skin System (visual overlays for sports scoreboards) — NOT SUPPORTED YET
-- Skins do not render with the current scoreboard plugins: the only hook is `SportsCore._render_game()` in `src/base_classes/sports/core.py`, and no current scoreboard plugin (monorepo or third-party registry) builds on `src.base_classes`
-- So core doesn't offer them: no Visual Skin dropdown (`get_plugin_schema` skips `inject_skin_selector`), the store hides/refuses `"type": "skin"` entries, `GET /api/v3/skins` reports `"supported": false`. Switch: `SKINS_RENDER_SUPPORTED` in `src/skin_system/__init__.py`
-- Stored `skin` / `skin_options` config values must keep loading and saving (base schema allows them; form saves deep-merge over the stored section)
-- Skins live in `skins/<skin-id>/` (skin.json + skin.py), NOT in plugin dirs — plugin reinstall deletes plugin dirs
-- Core: `src/skin_system/` (ScoreboardSkin, SkinContext, runtime); keep it and its tests
-- Skins render onto `ctx.canvas` only; fallback to built-in renderer on `False`/exception (3 strikes disables for session)
-- View-model guaranteed keys are frozen (see `test/test_skin_system.py::TestViewModelContract`) — renaming keys in `_extract_game_details_common` or sport extractors breaks published skins
-- Validate skins headlessly: `python scripts/validate_skin.py --skin <id>`; docs: `docs/SKIN_SYSTEM.md`, `docs/CREATING_SKINS.md`
-- Skins are NOT monorepo plugins: no manifest bump / update_registry.py needed
-
 ## Common Pitfalls
 - paho-mqtt 2.x needs `callback_api_version=mqtt.CallbackAPIVersion.VERSION1` for v1 compat
 - BasePlugin uses `get_logger()` from `src.logging_config`, not standard `logging.getLogger()`