Clean up repo: gitignore binaries, remove stale docs

- Add compiled binaries (grapho, *_test) to .gitignore
- Remove grapho binary from tracking (build from source)
- Delete obsolete LUX-LIMITATIONS.md
- Minor README wording fix

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -3,6 +3,10 @@ result
 result-*
 .direnv/
 
+# Compiled binaries
+grapho
+*_test
+
 # Secrets (NEVER commit unencrypted secrets)
 secrets/*.yaml
 !secrets/secrets.yaml.example