Ln 770 Crosscutting Setup

This is a coordinator skill that sets up all the standard backend plumbing in one go: structured logging with Serilog or structlog, global error handlers, CORS policies, health check endpoints, and API documentation. It detects whether you're working with .NET or Python/FastAPI, checks what's already configured so it won't duplicate anything, then delegates to five worker skills to handle each concern. The nice part is it generates a single aggregation file (ServiceExtensions.cs or middleware/__init__.py) so you have one place to wire everything up. Best used at project start or when inheriting a backend that's missing these baseline concerns. Idempotent by design, so safe to rerun.