Troubleshoot Design Agent

Report missing settings by name and never paste a key, token or .env line.

1. uv run design-agent doctor (add --json to parse it) checks every model role, keys, the brand, the sandbox, MDA declarations, MCP and Paper. Fix what it names first.
2. Real calls, a few cents each: uv run design-agent test model, test sandbox, test paper, test slack, test db.
3. Match the symptom: