chore: make contributor tooling work; fix drifted docs (#648)

- mypy.ini parses again (multi-line exclude and inline value comments made
  mypy reject the file); the mypy pre-commit hook is manual-only until the
  ~500 existing errors in src/ are paid down, and CONTRIBUTING says so
- .gitignore: ignore all of config/ except the templates (ytm_auth.json and
  others weren't ignored)
- .gitattributes: LF for .sh and .service
- claude-code-review: skip fork PRs, which have no secrets
- check_system_compatibility.sh: 3.13 supported, <3.10 an error
- docs/scripts drift: emulator guide, README API Metrics, route count,
  docs index, scripts README; pyflakes nits in dev scripts

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

docs/README.md

@@ -56,6 +56,8 @@ Going deeper:
 - [ADVANCED_FEATURES.md](ADVANCED_FEATURES.md) — Vegas scroll, on-demand display,
   cache management, background services, permissions
 - [FONT_MANAGER.md](FONT_MANAGER.md) — font system
+- [SCROLL_PERFORMANCE.md](SCROLL_PERFORMANCE.md) — how scrolling is paced, and how to make a plugin's marquee smooth
+- [OFFSCREEN_RENDERING.md](OFFSCREEN_RENDERING.md) — rendering plugin content off the render thread
 - [PERMISSIONS.md](PERMISSIONS.md) — file ownership, sudo rules, repair scripts
 - [MQTT bridge](../integrations/mqtt_bridge/README.md) — control the display from Home Assistant over MQTT