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>

scripts/dev_server.py

@@ -423,7 +423,7 @@ def main():
     global _extra_dirs
     _extra_dirs = args.extra_dir
 
-    print(f"LEDMatrix Dev Preview Server")
+    print("LEDMatrix Dev Preview Server")
     print(f"Open http://{args.host}:{args.port} in your browser")
     print(f"Plugin search dirs: {[str(d) for d in get_search_dirs()]}")
     print()